path: root/src/VBox/Storage/QCOW.cpp

/* $Id: QCOW.cpp $ */
/** @file
 * QCOW - QCOW Disk image.
 */

/*
 * Copyright (C) 2011-2023 Oracle and/or its affiliates.
 *
 * This file is part of VirtualBox base platform packages, as
 * available from https://www.virtualbox.org.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation, in version 3 of the
 * License.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, see <https://www.gnu.org/licenses>.
 *
 * SPDX-License-Identifier: GPL-3.0-only
 */


/*********************************************************************************************************************************
*   Header Files                                                                                                                 *
*********************************************************************************************************************************/
#define LOG_GROUP LOG_GROUP_VD_QCOW
#include <VBox/vd-plugin.h>
#include <VBox/err.h>

#include <VBox/log.h>
#include <iprt/asm.h>
#include <iprt/assert.h>
#include <iprt/string.h>
#include <iprt/alloc.h>
#include <iprt/path.h>
#include <iprt/list.h>
#include <iprt/zip.h>

#include "VDBackends.h"
#include "VDBackendsInline.h"

/** @page pg_storage_qcow   QCOW Storage Backend
 * The QCOW backend implements support for the qemu copy on write format (short QCOW).
 *
 * The official specification for qcow is available at
 * https://github.com/qemu/qemu/blob/master/docs/interop/qcow2.txt version 2 and 3.
 * For version 1 there is no official specification available but the format is described
 * at http://people.gnome.org/~markmc/qcow-image-format-version-1.html.
 *
 * Missing things to implement:
 *    - v2 image creation and handling of the reference count table. (Blocker to enable support for V2 images)
 *    - cluster encryption
 *    - cluster compression
 *    - compaction
 *    - resizing
 */


/*********************************************************************************************************************************
*   Structures in a QCOW image, big endian                                                                                       *
*********************************************************************************************************************************/

#pragma pack(1) /* Completely unnecessary. */
typedef struct QCowHeader
{
    /** Magic value. */
    uint32_t    u32Magic;
    /** Version of the image. */
    uint32_t    u32Version;
    /** Version dependent data. */
    union
    {
        /** Version 1. */
        struct
        {
            /** Backing file offset. */
            uint64_t    u64BackingFileOffset;
            /** Size of the backing file. */
            uint32_t    u32BackingFileSize;
            /** mtime (Modification time?) - can be ignored. */
            uint32_t    u32MTime;
            /** Logical size of the image in bytes. */
            uint64_t    u64Size;
            /** Number of bits in the virtual offset used as a cluster offset. */
            uint8_t     u8ClusterBits;
            /** Number of bits in the virtual offset used for the L2 index. */
            uint8_t     u8L2Bits;
            /** Padding because the header is not packed in the original source. */
            uint16_t    u16Padding;
            /** Used cryptographic method. */
            uint32_t    u32CryptMethod;
            /** Offset of the L1 table in the image in bytes. */
            uint64_t    u64L1TableOffset;
        } v1;
        /** Version 2 (and also containing extensions for version 3). */
        struct
        {
            /** Backing file offset. */
            uint64_t    u64BackingFileOffset;
            /** Size of the backing file. */
            uint32_t    u32BackingFileSize;
            /** Number of bits in the virtual offset used as a cluster offset. */
            uint32_t    u32ClusterBits;
            /** Logical size of the image. */
            uint64_t    u64Size;
            /** Used cryptographic method. */
            uint32_t    u32CryptMethod;
            /** Size of the L1 table in entries (each 8bytes big). */
            uint32_t    u32L1Size;
            /** Offset of the L1 table in the image in bytes. */
            uint64_t    u64L1TableOffset;
            /** Start of the refcount table in the image. */
            uint64_t    u64RefcountTableOffset;
            /** Size of the refcount table in clusters. */
            uint32_t    u32RefcountTableClusters;
            /** Number of snapshots in the image. */
            uint32_t    u32NbSnapshots;
            /** Offset of the first snapshot header in the image. */
            uint64_t    u64SnapshotsOffset;
            /** Version 3 additional data. */
            struct
            {
                /** Incompatible features. */
                uint64_t    u64IncompatFeat;
                /** Compatible features. */
                uint64_t    u64CompatFeat;
                /** Autoclear features. */
                uint64_t    u64AutoClrFeat;
                /** Width in bits of a reference count block. */
                uint32_t    u32RefCntWidth;
                /** Lenght of the header structure in bytes (for the header extensions). */
                uint32_t    u32HdrLenBytes;
            } v3;
        } v2;
    } Version;
} QCowHeader;
#pragma pack()
/** Pointer to a on disk QCOW header. */
typedef QCowHeader *PQCowHeader;

/** QCOW magic value. */
#define QCOW_MAGIC                            UINT32_C(0x514649fb) /* QFI\0xfb */
/** Size of the V1 header. */
#define QCOW_V1_HDR_SIZE                      (48)
/** Size of the V2 header. */
#define QCOW_V2_HDR_SIZE                      (72)

/** Cluster is compressed flag for QCOW images. */
#define QCOW_V1_COMPRESSED_FLAG               RT_BIT_64(63)

/** Copied flag for QCOW2 images. */
#define QCOW_V2_COPIED_FLAG                   RT_BIT_64(63)
/** Cluster is compressed flag for QCOW2 images. */
#define QCOW_V2_COMPRESSED_FLAG               RT_BIT_64(62)
/** The mask for extracting the offset from either the L1 or L2 table. */
#define QCOW_V2_TBL_OFFSET_MASK               UINT64_C(0x00fffffffffffe00)

/** Incompatible feature: Dirty bit, reference count may be inconsistent. */
#define QCOW_V3_INCOMPAT_FEAT_F_DIRTY         RT_BIT_64(0)
/** Incompatible feature: Image is corrupt and needs repair. */
#define QCOW_V3_INCOMPAT_FEAT_F_CORRUPT       RT_BIT_64(1)
/** Incompatible feature: External data file. */
#define QCOW_V3_INCOMPAT_FEAT_F_EXTERNAL_DATA RT_BIT_64(2)
/** The incompatible features we support currently. */
#define QCOW_V3_INCOMPAT_FEAT_SUPPORTED_MASK  UINT64_C(0x0)

/** Compatible feature: Lazy reference counters. */
#define QCOW_V3_COMPAT_FEAT_F_LAZY_REF_COUNT  RT_BIT_64(0)
/** The compatible features we support currently. */
#define QCOW_V3_COMPAT_FEAT_SUPPORTED_MASK    UINT64_C(0x0)

/** Auto clear feature: Bitmaps extension. */
#define QCOW_V3_AUTOCLR_FEAT_F_BITMAPS        RT_BIT_64(0)
/** Auto clear feature: The external data file is raw image which can be accessed standalone. */
#define QCOW_V3_AUTOCLR_FEAT_F_EXT_RAW_DATA   RT_BIT_64(1)
/** The autoclear features we support currently. */
#define QCOW_V3_AUTOCLR_FEAT_SUPPORTED_MASK   UINT64_C(0x0)


/*********************************************************************************************************************************
*   Constants And Macros, Structures and Typedefs                                                                                *
*********************************************************************************************************************************/

/**
 * QCOW L2 cache entry.
 */
typedef struct QCOWL2CACHEENTRY
{
    /** List node for the search list. */
    RTLISTNODE              NodeSearch;
    /** List node for the LRU list. */
    RTLISTNODE              NodeLru;
    /** Reference counter. */
    uint32_t                cRefs;
    /** The offset of the L2 table, used as search key. */
    uint64_t                offL2Tbl;
    /** Pointer to the cached L2 table. */
    uint64_t               *paL2Tbl;
} QCOWL2CACHEENTRY, *PQCOWL2CACHEENTRY;

/** Maximum amount of memory the cache is allowed to use. */
#define QCOW_L2_CACHE_MEMORY_MAX (2*_1M)

/** QCOW default cluster size for image version 2. */
#define QCOW2_CLUSTER_SIZE_DEFAULT (64*_1K)
/** QCOW default cluster size for image version 1. */
#define QCOW_CLUSTER_SIZE_DEFAULT (4*_1K)
/** QCOW default L2 table size in clusters. */
#define QCOW_L2_CLUSTERS_DEFAULT (1)

/**
 * QCOW image data structure.
 */
typedef struct QCOWIMAGE
{
    /** Image name. */
    const char          *pszFilename;
    /** Storage handle. */
    PVDIOSTORAGE        pStorage;

    /** Pointer to the per-disk VD interface list. */
    PVDINTERFACE        pVDIfsDisk;
    /** Pointer to the per-image VD interface list. */
    PVDINTERFACE        pVDIfsImage;
    /** Error interface. */
    PVDINTERFACEERROR   pIfError;
    /** I/O interface. */
    PVDINTERFACEIOINT   pIfIo;

    /** Open flags passed by VBoxHD layer. */
    unsigned            uOpenFlags;
    /** Image flags defined during creation or determined during open. */
    unsigned            uImageFlags;
    /** Total size of the image. */
    uint64_t            cbSize;
    /** Physical geometry of this image. */
    VDGEOMETRY          PCHSGeometry;
    /** Logical geometry of this image. */
    VDGEOMETRY          LCHSGeometry;

    /** Image version. */
    unsigned            uVersion;
    /** MTime field - used only to preserve value in opened images, unmodified otherwise. */
    uint32_t            MTime;

    /** Filename of the backing file if any. */
    char               *pszBackingFilename;
    /** Offset of the filename in the image. */
    uint64_t            offBackingFilename;
    /** Size of the backing filename excluding \0. */
    uint32_t            cbBackingFilename;

    /** Next offset of a new cluster, aligned to sector size. */
    uint64_t            offNextCluster;
    /** Cluster size in bytes. */
    uint32_t            cbCluster;
    /** Number of bits in the virtual offset used as the cluster offset. */
    uint32_t            cClusterBits;
    /** Bitmask to extract the offset from a compressed cluster descriptor. */
    uint64_t            fMaskCompressedClusterOffset;
    /** Bitmask to extract the sector count from a compressed cluster descriptor. */
    uint64_t            fMaskCompressedClusterSectors;
    /** Number of bits to shift the sector count to the right to get the final value. */
    uint32_t            cBitsShiftRCompressedClusterSectors;
    /** Number of entries in the L1 table. */
    uint32_t            cL1TableEntries;
    /** Size of an L1 rounded to the next cluster size. */
    uint32_t            cbL1Table;
    /** Pointer to the L1 table. */
    uint64_t            *paL1Table;
    /** Offset of the L1 table. */
    uint64_t            offL1Table;

    /** Size of the L2 table in bytes. */
    uint32_t            cbL2Table;
    /** Number of entries in the L2 table. */
    uint32_t            cL2TableEntries;
    /** Memory occupied by the L2 table cache. */
    size_t              cbL2Cache;
    /** The sorted L2 entry list used for searching. */
    RTLISTNODE          ListSearch;
    /** The LRU L2 entry list used for eviction. */
    RTLISTNODE          ListLru;

    /** Offset of the refcount table. */
    uint64_t            offRefcountTable;
    /** Size of the refcount table in bytes. */
    uint32_t            cbRefcountTable;
    /** Number of entries in the refcount table. */
    uint32_t            cRefcountTableEntries;
    /** Pointer to the refcount table. */
    uint64_t           *paRefcountTable;

    /** Offset mask for a cluster. */
    uint64_t            fOffsetMask;
    /** Number of bits to shift to get the L1 index. */
    uint32_t            cL1Shift;
    /** L2 table mask to get the L2 index. */
    uint64_t            fL2Mask;
    /** Number of bits to shift to get the L2 index. */
    uint32_t            cL2Shift;

    /** Size of compressed cluster buffer. */
    size_t              cbCompCluster;
    /** Compressed cluster buffer. */
    void                *pvCompCluster;
    /** Buffer to hold the uncompressed data. */
    void                *pvCluster;

    /** Pointer to the L2 table we are currently allocating
     * (can be only one at a time). */
    PQCOWL2CACHEENTRY   pL2TblAlloc;
    /** The static region list. */
    VDREGIONLIST        RegionList;
} QCOWIMAGE, *PQCOWIMAGE;

/**
 * State of the async cluster allocation.
 */
typedef enum QCOWCLUSTERASYNCALLOCSTATE
{
    /** Invalid. */
    QCOWCLUSTERASYNCALLOCSTATE_INVALID = 0,
    /** L2 table allocation. */
    QCOWCLUSTERASYNCALLOCSTATE_L2_ALLOC,
    /** Link L2 table into L1. */
    QCOWCLUSTERASYNCALLOCSTATE_L2_LINK,
    /** Allocate user data cluster. */
    QCOWCLUSTERASYNCALLOCSTATE_USER_ALLOC,
    /** Link user data cluster. */
    QCOWCLUSTERASYNCALLOCSTATE_USER_LINK,
    /** 32bit blowup. */
    QCOWCLUSTERASYNCALLOCSTATE_32BIT_HACK = 0x7fffffff
} QCOWCLUSTERASYNCALLOCSTATE, *PQCOWCLUSTERASYNCALLOCSTATE;

/**
 * Data needed to track async cluster allocation.
 */
typedef struct QCOWCLUSTERASYNCALLOC
{
    /** The state of the cluster allocation. */
    QCOWCLUSTERASYNCALLOCSTATE enmAllocState;
    /** Old image size to rollback in case of an error. */
    uint64_t                   offNextClusterOld;
    /** L1 index to link if any. */
    uint32_t                   idxL1;
    /** L2 index to link, required in any case. */
    uint32_t                   idxL2;
    /** Start offset of the allocated cluster. */
    uint64_t                   offClusterNew;
    /** L2 cache entry if a L2 table is allocated. */
    PQCOWL2CACHEENTRY          pL2Entry;
    /** Number of bytes to write. */
    size_t                     cbToWrite;
} QCOWCLUSTERASYNCALLOC, *PQCOWCLUSTERASYNCALLOC;


/*********************************************************************************************************************************
*   Static Variables                                                                                                             *
*********************************************************************************************************************************/

/** NULL-terminated array of supported file extensions. */
static const VDFILEEXTENSION s_aQCowFileExtensions[] =
{
    {"qcow", VDTYPE_HDD},
    {"qcow2", VDTYPE_HDD},
    {NULL,  VDTYPE_INVALID}
};


/*********************************************************************************************************************************
*   Internal Functions                                                                                                           *
*********************************************************************************************************************************/

/**
 * Return power of 2 or 0 if num error.
 *
 * @returns The power of 2 or 0 if the given number is not a power of 2.
 * @param   u32    The number.
 */
static uint32_t qcowGetPowerOfTwo(uint32_t u32)
{
    if (u32 == 0)
        return 0;
    uint32_t uPower2 = 0;
    while ((u32 & 1) == 0)
    {
        u32 >>= 1;
        uPower2++;
    }
    return u32 == 1 ? uPower2 : 0;
}


/**
 * Converts the image header to the host endianess and performs basic checks.
 *
 * @returns Whether the given header is valid or not.
 * @param   pHeader    Pointer to the header to convert.
 */
static bool qcowHdrConvertToHostEndianess(PQCowHeader pHeader)
{
    pHeader->u32Magic                                = RT_BE2H_U32(pHeader->u32Magic);
    pHeader->u32Version                              = RT_BE2H_U32(pHeader->u32Version);

    if (pHeader->u32Magic != QCOW_MAGIC)
        return false;

    if (pHeader->u32Version == 1)
    {
        pHeader->Version.v1.u64BackingFileOffset     = RT_BE2H_U64(pHeader->Version.v1.u64BackingFileOffset);
        pHeader->Version.v1.u32BackingFileSize       = RT_BE2H_U32(pHeader->Version.v1.u32BackingFileSize);
        pHeader->Version.v1.u32MTime                 = RT_BE2H_U32(pHeader->Version.v1.u32MTime);
        pHeader->Version.v1.u64Size                  = RT_BE2H_U64(pHeader->Version.v1.u64Size);
        pHeader->Version.v1.u32CryptMethod           = RT_BE2H_U32(pHeader->Version.v1.u32CryptMethod);
        pHeader->Version.v1.u64L1TableOffset         = RT_BE2H_U64(pHeader->Version.v1.u64L1TableOffset);
    }
    else if (pHeader->u32Version == 2 || pHeader->u32Version == 3)
    {
        pHeader->Version.v2.u64BackingFileOffset     = RT_BE2H_U64(pHeader->Version.v2.u64BackingFileOffset);
        pHeader->Version.v2.u32BackingFileSize       = RT_BE2H_U32(pHeader->Version.v2.u32BackingFileSize);
        pHeader->Version.v2.u32ClusterBits           = RT_BE2H_U32(pHeader->Version.v2.u32ClusterBits);
        pHeader->Version.v2.u64Size                  = RT_BE2H_U64(pHeader->Version.v2.u64Size);
        pHeader->Version.v2.u32CryptMethod           = RT_BE2H_U32(pHeader->Version.v2.u32CryptMethod);
        pHeader->Version.v2.u32L1Size                = RT_BE2H_U32(pHeader->Version.v2.u32L1Size);
        pHeader->Version.v2.u64L1TableOffset         = RT_BE2H_U64(pHeader->Version.v2.u64L1TableOffset);
        pHeader->Version.v2.u64RefcountTableOffset   = RT_BE2H_U64(pHeader->Version.v2.u64RefcountTableOffset);
        pHeader->Version.v2.u32RefcountTableClusters = RT_BE2H_U32(pHeader->Version.v2.u32RefcountTableClusters);
        pHeader->Version.v2.u32NbSnapshots           = RT_BE2H_U32(pHeader->Version.v2.u32NbSnapshots);
        pHeader->Version.v2.u64SnapshotsOffset       = RT_BE2H_U64(pHeader->Version.v2.u64SnapshotsOffset);

        if (pHeader->u32Version == 3)
        {
            pHeader->Version.v2.v3.u64IncompatFeat   = RT_BE2H_U64(pHeader->Version.v2.v3.u64IncompatFeat);
            pHeader->Version.v2.v3.u64CompatFeat     = RT_BE2H_U64(pHeader->Version.v2.v3.u64CompatFeat);
            pHeader->Version.v2.v3.u64AutoClrFeat    = RT_BE2H_U64(pHeader->Version.v2.v3.u64AutoClrFeat);
            pHeader->Version.v2.v3.u32RefCntWidth    = RT_BE2H_U32(pHeader->Version.v2.v3.u32RefCntWidth);
            pHeader->Version.v2.v3.u32HdrLenBytes    = RT_BE2H_U32(pHeader->Version.v2.v3.u32HdrLenBytes);
        }
    }
    else
        return false;

    return true;
}

/**
 * Creates a QCOW header from the given image state.
 *
 * @param   pImage     Image instance data.
 * @param   pHeader    Pointer to the header to convert.
 * @param   pcbHeader  Where to store the size of the header to write.
 */
static void qcowHdrConvertFromHostEndianess(PQCOWIMAGE pImage, PQCowHeader pHeader,
                                            size_t *pcbHeader)
{
    memset(pHeader, 0, sizeof(QCowHeader));

    pHeader->u32Magic                                = RT_H2BE_U32(QCOW_MAGIC);
    pHeader->u32Version                              = RT_H2BE_U32(pImage->uVersion);
    if (pImage->uVersion == 1)
    {
        pHeader->Version.v1.u64BackingFileOffset     = RT_H2BE_U64(pImage->offBackingFilename);
        pHeader->Version.v1.u32BackingFileSize       = RT_H2BE_U32(pImage->cbBackingFilename);
        pHeader->Version.v1.u32MTime                 = RT_H2BE_U32(pImage->MTime);
        pHeader->Version.v1.u64Size                  = RT_H2BE_U64(pImage->cbSize);
        pHeader->Version.v1.u8ClusterBits            = (uint8_t)qcowGetPowerOfTwo(pImage->cbCluster);
        pHeader->Version.v1.u8L2Bits                 = (uint8_t)qcowGetPowerOfTwo(pImage->cL2TableEntries);
        pHeader->Version.v1.u32CryptMethod           = RT_H2BE_U32(0);
        pHeader->Version.v1.u64L1TableOffset         = RT_H2BE_U64(pImage->offL1Table);
        *pcbHeader = QCOW_V1_HDR_SIZE;
    }
    else if (pImage->uVersion == 2)
    {
        pHeader->Version.v2.u64BackingFileOffset     = RT_H2BE_U64(pImage->offBackingFilename);
        pHeader->Version.v2.u32BackingFileSize       = RT_H2BE_U32(pImage->cbBackingFilename);
        pHeader->Version.v2.u32ClusterBits           = RT_H2BE_U32(qcowGetPowerOfTwo(pImage->cbCluster));
        pHeader->Version.v2.u64Size                  = RT_H2BE_U64(pImage->cbSize);
        pHeader->Version.v2.u32CryptMethod           = RT_H2BE_U32(0);
        pHeader->Version.v2.u32L1Size                = RT_H2BE_U32(pImage->cL1TableEntries);
        pHeader->Version.v2.u64L1TableOffset         = RT_H2BE_U64(pImage->offL1Table);
        pHeader->Version.v2.u64RefcountTableOffset   = RT_H2BE_U64(pImage->offRefcountTable);
        pHeader->Version.v2.u32RefcountTableClusters = RT_H2BE_U32(pImage->cbRefcountTable / pImage->cbCluster);
        pHeader->Version.v2.u32NbSnapshots           = RT_H2BE_U32(0);
        pHeader->Version.v2.u64SnapshotsOffset       = RT_H2BE_U64((uint64_t)0);
        *pcbHeader = QCOW_V2_HDR_SIZE;
    }
    else
        AssertMsgFailed(("Invalid version of the QCOW image format %d\n", pImage->uVersion));
}

/**
 * Convert table entries from little endian to host endianess.
 *
 * @param   paTbl       Pointer to the table.
 * @param   cEntries    Number of entries in the table.
 */
static void qcowTableConvertToHostEndianess(uint64_t *paTbl, uint32_t cEntries)
{
    while (cEntries-- > 0)
    {
        *paTbl = RT_BE2H_U64(*paTbl);
        paTbl++;
    }
}

/**
 * Convert table entries from host to little endian format.
 *
 * @param   paTblImg    Pointer to the table which will store the little endian table.
 * @param   paTbl       The source table to convert.
 * @param   cEntries    Number of entries in the table.
 */
static void qcowTableConvertFromHostEndianess(uint64_t *paTblImg, uint64_t const *paTbl,
                                              uint32_t cEntries)
{
    while (cEntries-- > 0)
    {
        *paTblImg = RT_H2BE_U64(*paTbl);
        paTbl++;
        paTblImg++;
    }
}

/**
 * Creates the L2 table cache.
 *
 * @returns VBox status code.
 * @param   pImage    The image instance data.
 */
static int qcowL2TblCacheCreate(PQCOWIMAGE pImage)
{
    pImage->cbL2Cache = 0;
    RTListInit(&pImage->ListSearch);
    RTListInit(&pImage->ListLru);

    return VINF_SUCCESS;
}

/**
 * Destroys the L2 table cache.
 *
 * @param   pImage    The image instance data.
 */
static void qcowL2TblCacheDestroy(PQCOWIMAGE pImage)
{
    PQCOWL2CACHEENTRY pL2Entry;
    PQCOWL2CACHEENTRY pL2Next;
    RTListForEachSafe(&pImage->ListSearch, pL2Entry, pL2Next, QCOWL2CACHEENTRY, NodeSearch)
    {
        Assert(!pL2Entry->cRefs);

        RTListNodeRemove(&pL2Entry->NodeSearch);
        RTMemPageFree(pL2Entry->paL2Tbl, pImage->cbL2Table);
        RTMemFree(pL2Entry);
    }

    pImage->cbL2Cache       = 0;
    RTListInit(&pImage->ListSearch);
    RTListInit(&pImage->ListLru);
}

/**
 * Returns the L2 table matching the given offset or NULL if none could be found.
 *
 * @returns Pointer to the L2 table cache entry or NULL.
 * @param   pImage    The image instance data.
 * @param   offL2Tbl  Offset of the L2 table to search for.
 */
static PQCOWL2CACHEENTRY qcowL2TblCacheRetain(PQCOWIMAGE pImage, uint64_t offL2Tbl)
{
    if (   pImage->pL2TblAlloc
        && pImage->pL2TblAlloc->offL2Tbl == offL2Tbl)
    {
        pImage->pL2TblAlloc->cRefs++;
        return pImage->pL2TblAlloc;
    }

    PQCOWL2CACHEENTRY pL2Entry;
    RTListForEach(&pImage->ListSearch, pL2Entry, QCOWL2CACHEENTRY, NodeSearch)
    {
        if (pL2Entry->offL2Tbl == offL2Tbl)
            break;
    }

    if (!RTListNodeIsDummy(&pImage->ListSearch, pL2Entry, QCOWL2CACHEENTRY, NodeSearch))
    {
        /* Update LRU list. */
        RTListNodeRemove(&pL2Entry->NodeLru);
        RTListPrepend(&pImage->ListLru, &pL2Entry->NodeLru);
        pL2Entry->cRefs++;
        return pL2Entry;
    }

    return NULL;
}

/**
 * Releases a L2 table cache entry.
 *
 * @param   pL2Entry    The L2 cache entry.
 */
static void qcowL2TblCacheEntryRelease(PQCOWL2CACHEENTRY pL2Entry)
{
    Assert(pL2Entry->cRefs > 0);
    pL2Entry->cRefs--;
}

/**
 * Allocates a new L2 table from the cache evicting old entries if required.
 *
 * @returns Pointer to the L2 cache entry or NULL.
 * @param   pImage    The image instance data.
 */
static PQCOWL2CACHEENTRY qcowL2TblCacheEntryAlloc(PQCOWIMAGE pImage)
{
    PQCOWL2CACHEENTRY pL2Entry = NULL;

    if (pImage->cbL2Cache + pImage->cbL2Table <= QCOW_L2_CACHE_MEMORY_MAX)
    {
        /* Add a new entry. */
        pL2Entry = (PQCOWL2CACHEENTRY)RTMemAllocZ(sizeof(QCOWL2CACHEENTRY));
        if (pL2Entry)
        {
            pL2Entry->paL2Tbl = (uint64_t *)RTMemPageAllocZ(pImage->cbL2Table);
            if (RT_UNLIKELY(!pL2Entry->paL2Tbl))
            {
                RTMemFree(pL2Entry);
                pL2Entry = NULL;
            }
            else
            {
                pL2Entry->cRefs    = 1;
                pImage->cbL2Cache += pImage->cbL2Table;
            }
        }
    }
    else
    {
        /* Evict the last not in use entry and use it */
        Assert(!RTListIsEmpty(&pImage->ListLru));

        RTListForEachReverse(&pImage->ListLru, pL2Entry, QCOWL2CACHEENTRY, NodeLru)
        {
            if (!pL2Entry->cRefs)
                break;
        }

        if (!RTListNodeIsDummy(&pImage->ListSearch, pL2Entry, QCOWL2CACHEENTRY, NodeSearch))
        {
            RTListNodeRemove(&pL2Entry->NodeSearch);
            RTListNodeRemove(&pL2Entry->NodeLru);
            pL2Entry->offL2Tbl = 0;
            pL2Entry->cRefs    = 1;
        }
        else
            pL2Entry = NULL;
    }

    return pL2Entry;
}

/**
 * Frees a L2 table cache entry.
 *
 * @param   pImage    The image instance data.
 * @param   pL2Entry  The L2 cache entry to free.
 */
static void qcowL2TblCacheEntryFree(PQCOWIMAGE pImage, PQCOWL2CACHEENTRY pL2Entry)
{
    Assert(!pL2Entry->cRefs);
    RTMemPageFree(pL2Entry->paL2Tbl, pImage->cbL2Table);
    RTMemFree(pL2Entry);

    pImage->cbL2Cache -= pImage->cbL2Table;
}

/**
 * Inserts an entry in the L2 table cache.
 *
 * @param   pImage    The image instance data.
 * @param   pL2Entry  The L2 cache entry to insert.
 */
static void qcowL2TblCacheEntryInsert(PQCOWIMAGE pImage, PQCOWL2CACHEENTRY pL2Entry)
{
    Assert(pL2Entry->offL2Tbl > 0);

    /* Insert at the top of the LRU list. */
    RTListPrepend(&pImage->ListLru, &pL2Entry->NodeLru);

    if (RTListIsEmpty(&pImage->ListSearch))
    {
        RTListAppend(&pImage->ListSearch, &pL2Entry->NodeSearch);
    }
    else
    {
        /* Insert into search list. */
        PQCOWL2CACHEENTRY pIt;
        pIt = RTListGetFirst(&pImage->ListSearch, QCOWL2CACHEENTRY, NodeSearch);
        if (pIt->offL2Tbl > pL2Entry->offL2Tbl)
            RTListPrepend(&pImage->ListSearch, &pL2Entry->NodeSearch);
        else
        {
            bool fInserted = false;

            RTListForEach(&pImage->ListSearch, pIt, QCOWL2CACHEENTRY, NodeSearch)
            {
                Assert(pIt->offL2Tbl != pL2Entry->offL2Tbl);
                if (pIt->offL2Tbl < pL2Entry->offL2Tbl)
                {
                    RTListNodeInsertAfter(&pIt->NodeSearch, &pL2Entry->NodeSearch);
                    fInserted = true;
                    break;
                }
            }
            Assert(fInserted);
        }
    }
}

/**
 * Fetches the L2 from the given offset trying the LRU cache first and
 * reading it from the image after a cache miss.
 *
 * @returns VBox status code.
 * @param   pImage    Image instance data.
 * @param   pIoCtx    The I/O context.
 * @param   offL2Tbl  The offset of the L2 table in the image.
 * @param   ppL2Entry Where to store the L2 table on success.
 */
static int qcowL2TblCacheFetch(PQCOWIMAGE pImage, PVDIOCTX pIoCtx, uint64_t offL2Tbl,
                               PQCOWL2CACHEENTRY *ppL2Entry)
{
    int rc = VINF_SUCCESS;

    /* Try to fetch the L2 table from the cache first. */
    PQCOWL2CACHEENTRY pL2Entry = qcowL2TblCacheRetain(pImage, offL2Tbl);
    if (!pL2Entry)
    {
        pL2Entry = qcowL2TblCacheEntryAlloc(pImage);

        if (pL2Entry)
        {
            /* Read from the image. */
            PVDMETAXFER pMetaXfer;

            pL2Entry->offL2Tbl = offL2Tbl;
            rc = vdIfIoIntFileReadMeta(pImage->pIfIo, pImage->pStorage,
                                       offL2Tbl, pL2Entry->paL2Tbl,
                                       pImage->cbL2Table, pIoCtx,
                                       &pMetaXfer, NULL, NULL);
            if (RT_SUCCESS(rc))
            {
                vdIfIoIntMetaXferRelease(pImage->pIfIo, pMetaXfer);
#if defined(RT_LITTLE_ENDIAN)
                qcowTableConvertToHostEndianess(pL2Entry->paL2Tbl, pImage->cL2TableEntries);
#endif
                qcowL2TblCacheEntryInsert(pImage, pL2Entry);
            }
            else
            {
                qcowL2TblCacheEntryRelease(pL2Entry);
                qcowL2TblCacheEntryFree(pImage, pL2Entry);
            }
        }
        else
            rc = VERR_NO_MEMORY;
    }

    if (RT_SUCCESS(rc))
        *ppL2Entry = pL2Entry;

    return rc;
}

/**
 * Sets the L1, L2 and offset bitmasks and L1 and L2 bit shift members.
 *
 * @param   pImage    The image instance data.
 */
static void qcowTableMasksInit(PQCOWIMAGE pImage)
{
    uint32_t cClusterBits, cL2TableBits;

    cClusterBits = qcowGetPowerOfTwo(pImage->cbCluster);
    cL2TableBits = qcowGetPowerOfTwo(pImage->cL2TableEntries);

    Assert(cClusterBits + cL2TableBits < 64);

    pImage->fOffsetMask = ((uint64_t)pImage->cbCluster - 1);
    pImage->fL2Mask     = ((uint64_t)pImage->cL2TableEntries - 1) << cClusterBits;
    pImage->cL2Shift    = cClusterBits;
    pImage->cL1Shift    = cClusterBits + cL2TableBits;
}

/**
 * Converts a given logical offset into the
 *
 * @param   pImage         The image instance data.
 * @param   off            The logical offset to convert.
 * @param   pidxL1         Where to store the index in the L1 table on success.
 * @param   pidxL2         Where to store the index in the L2 table on success.
 * @param   poffCluster    Where to store the offset in the cluster on success.
 */
DECLINLINE(void) qcowConvertLogicalOffset(PQCOWIMAGE pImage, uint64_t off, uint32_t *pidxL1,
                                          uint32_t *pidxL2, uint32_t *poffCluster)
{
    AssertPtr(pidxL1);
    AssertPtr(pidxL2);
    AssertPtr(poffCluster);

    *poffCluster = off & pImage->fOffsetMask;
    *pidxL1      = off >> pImage->cL1Shift;
    *pidxL2      = (off & pImage->fL2Mask) >> pImage->cL2Shift;
}

/**
 * Converts Cluster size to a byte size.
 *
 * @returns Number of bytes derived from the given number of clusters.
 * @param   pImage    The image instance data.
 * @param   cClusters The clusters to convert.
 */
DECLINLINE(uint64_t) qcowCluster2Byte(PQCOWIMAGE pImage, uint64_t cClusters)
{
    return cClusters * pImage->cbCluster;
}

/**
 * Converts number of bytes to cluster size rounding to the next cluster.
 *
 * @returns Number of bytes derived from the given number of clusters.
 * @param   pImage    The image instance data.
 * @param   cb        Number of bytes to convert.
 */
DECLINLINE(uint64_t) qcowByte2Cluster(PQCOWIMAGE pImage, uint64_t cb)
{
    return cb / pImage->cbCluster + (cb % pImage->cbCluster ? 1 : 0);
}

/**
 * Allocates a new cluster in the image.
 *
 * @returns The start offset of the new cluster in the image.
 * @param   pImage    The image instance data.
 * @param   cClusters Number of clusters to allocate.
 */
DECLINLINE(uint64_t) qcowClusterAllocate(PQCOWIMAGE pImage, uint32_t cClusters)
{
    uint64_t offCluster;

    offCluster = pImage->offNextCluster;
    pImage->offNextCluster += cClusters*pImage->cbCluster;

    return offCluster;
}

/**
 * Returns the real image offset for a given cluster or an error if the cluster is not
 * yet allocated.
 *
 * @returns VBox status code.
 *          VERR_VD_BLOCK_FREE if the cluster is not yet allocated.
 * @param   pImage        The image instance data.
 * @param   pIoCtx        The I/O context.
 * @param   idxL1         The L1 index.
 * @param   idxL2         The L2 index.
 * @param   offCluster    Offset inside the cluster.
 * @param   poffImage     Where to store the image offset on success.
 * @param   pfCompressed  Where to store the flag whether the cluster is compressed on success.
 * @param   pcbCompressed Where to store the size of the compressed cluster in bytes on success.
 *                        Only valid when the cluster comrpessed flag is true.
 */
static int qcowConvertToImageOffset(PQCOWIMAGE pImage, PVDIOCTX pIoCtx,
                                    uint32_t idxL1, uint32_t idxL2,
                                    uint32_t offCluster, uint64_t *poffImage,
                                    bool *pfCompressed, size_t *pcbCompressed)
{
    int rc = VERR_VD_BLOCK_FREE;

    AssertReturn(idxL1 < pImage->cL1TableEntries, VERR_INVALID_PARAMETER);
    AssertReturn(idxL2 < pImage->cL2TableEntries, VERR_INVALID_PARAMETER);

    if (pImage->paL1Table[idxL1])
    {
        PQCOWL2CACHEENTRY pL2Entry;

        uint64_t offL2Tbl = pImage->paL1Table[idxL1];
        if (pImage->uVersion == 2)
            offL2Tbl &= QCOW_V2_TBL_OFFSET_MASK;
        rc = qcowL2TblCacheFetch(pImage, pIoCtx, offL2Tbl, &pL2Entry);
        if (RT_SUCCESS(rc))
        {
            /* Get real file offset. */
            if (pL2Entry->paL2Tbl[idxL2])
            {
                uint64_t off = pL2Entry->paL2Tbl[idxL2];

                /* Strip flags */
                if (pImage->uVersion == 2)
                {
                    if (RT_UNLIKELY(off & QCOW_V2_COMPRESSED_FLAG))
                    {
                        size_t cCompressedClusterSectors = ((off & pImage->fMaskCompressedClusterSectors) >> pImage->cBitsShiftRCompressedClusterSectors);
                        uint64_t offImage = off & pImage->fMaskCompressedClusterOffset;

                        *pfCompressed  = true;
                        *poffImage     = offImage;
                        *pcbCompressed = (cCompressedClusterSectors + 1) * 512 - (offImage & 511ULL);
                    }
                    else
                    {
                        off &= QCOW_V2_TBL_OFFSET_MASK;

                        *pfCompressed = false;
                        *poffImage = off + offCluster;
                    }
                }
                else
                {
                    if (RT_UNLIKELY(off & QCOW_V1_COMPRESSED_FLAG))
                    {
                        size_t cCompressedClusterSectors = (off & pImage->fMaskCompressedClusterSectors) >> pImage->cBitsShiftRCompressedClusterSectors;

                        *pfCompressed  = true;
                        *poffImage     = off & pImage->fMaskCompressedClusterOffset;
                        *pcbCompressed = cCompressedClusterSectors * 512; /* Only additional sectors */
                        /* Add remaining bytes of the sector the offset starts in. */
                        *pcbCompressed += 512 - RT_ALIGN_64(*poffImage, 512) - *poffImage;
                    }
                    else
                    {
                        off &= ~QCOW_V1_COMPRESSED_FLAG;

                        *pfCompressed = false;
                        *poffImage = off + offCluster;
                    }
                }
            }
            else
                rc = VERR_VD_BLOCK_FREE;

            qcowL2TblCacheEntryRelease(pL2Entry);
        }
    }

    return rc;
}

/**
 * Write the given table to image converting to the image endianess if required.
 *
 * @returns VBox status code.
 * @param   pImage        The image instance data.
 * @param   pIoCtx        The I/O context.
 * @param   offTbl        The offset the table should be written to.
 * @param   paTbl         The table to write.
 * @param   cbTbl         Size of the table in bytes.
 * @param   cTblEntries   Number entries in the table.
 * @param   pfnComplete   Callback called when the write completes.
 * @param   pvUser        Opaque user data to pass in the completion callback.
 */
static int qcowTblWrite(PQCOWIMAGE pImage, PVDIOCTX pIoCtx, uint64_t offTbl, uint64_t *paTbl,
                        size_t cbTbl, unsigned cTblEntries,
                        PFNVDXFERCOMPLETED pfnComplete, void *pvUser)
{
    int rc = VINF_SUCCESS;

#if defined(RT_LITTLE_ENDIAN)
    uint64_t *paTblImg = (uint64_t *)RTMemAllocZ(cbTbl);
    if (paTblImg)
    {
        qcowTableConvertFromHostEndianess(paTblImg, paTbl, cTblEntries);
        rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
                                    offTbl, paTblImg, cbTbl,
                                    pIoCtx, pfnComplete, pvUser);
        RTMemFree(paTblImg);
    }
    else
        rc = VERR_NO_MEMORY;
#else
    /* Write table directly. */
    RT_NOREF(cTblEntries);
    rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
                                offTbl, paTbl, cbTbl, pIoCtx,
                                pfnComplete, pvUser);
#endif

    return rc;
}

/**
 * Internal. Flush image data to disk.
 */
static int qcowFlushImage(PQCOWIMAGE pImage)
{
    int rc = VINF_SUCCESS;

    if (   pImage->pStorage
        && !(pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
        && pImage->cbL1Table)
    {
        QCowHeader Header;

#if defined(RT_LITTLE_ENDIAN)
        uint64_t *paL1TblImg = (uint64_t *)RTMemAllocZ(pImage->cbL1Table);
        if (paL1TblImg)
        {
            qcowTableConvertFromHostEndianess(paL1TblImg, pImage->paL1Table,
                                              pImage->cL1TableEntries);
            rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage,
                                        pImage->offL1Table, paL1TblImg,
                                        pImage->cbL1Table);
            RTMemFree(paL1TblImg);
        }
        else
            rc = VERR_NO_MEMORY;
#else
        /* Write L1 table directly. */
        rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage, pImage->offL1Table,
                                    pImage->paL1Table, pImage->cbL1Table);
#endif
        if (RT_SUCCESS(rc))
        {
            /* Write header. */
            size_t cbHeader = 0;
            qcowHdrConvertFromHostEndianess(pImage, &Header, &cbHeader);
            rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage, 0, &Header,
                                        cbHeader);
            if (RT_SUCCESS(rc))
                rc = vdIfIoIntFileFlushSync(pImage->pIfIo, pImage->pStorage);
        }
    }

    return rc;
}

/**
 * Internal. Free all allocated space for representing an image except pImage,
 * and optionally delete the image from disk.
 */
static int qcowFreeImage(PQCOWIMAGE pImage, bool fDelete)
{
    int rc = VINF_SUCCESS;

    /* Freeing a never allocated image (e.g. because the open failed) is
     * not signalled as an error. After all nothing bad happens. */
    if (pImage)
    {
        if (pImage->pStorage)
        {
            /* No point updating the file that is deleted anyway. */
            if (!fDelete)
                qcowFlushImage(pImage);

            rc = vdIfIoIntFileClose(pImage->pIfIo, pImage->pStorage);
            pImage->pStorage = NULL;
        }

        if (pImage->paRefcountTable)
            RTMemFree(pImage->paRefcountTable);
        pImage->paRefcountTable = NULL;

        if (pImage->paL1Table)
            RTMemFree(pImage->paL1Table);

        if (pImage->pszBackingFilename)
        {
            RTStrFree(pImage->pszBackingFilename);
            pImage->pszBackingFilename = NULL;
        }

        if (pImage->pvCompCluster)
        {
            RTMemFree(pImage->pvCompCluster);
            pImage->pvCompCluster = NULL;
            pImage->cbCompCluster = 0;
        }

        if (pImage->pvCluster)
        {
            RTMemFree(pImage->pvCluster);
            pImage->pvCluster = NULL;
        }

        qcowL2TblCacheDestroy(pImage);

        if (fDelete && pImage->pszFilename)
            vdIfIoIntFileDelete(pImage->pIfIo, pImage->pszFilename);
    }

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/**
 * Validates the header.
 *
 * @returns VBox status code.
 * @param   pImage    Image backend instance data.
 * @param   pHdr      The header to validate.
 * @param   cbFile    The image file size in bytes.
 */
static int qcowHdrValidate(PQCOWIMAGE pImage, PQCowHeader pHdr, uint64_t cbFile)
{
    if (pHdr->u32Version == 1)
    {
        /* Check that the backing filename is contained in the file. */
        if (pHdr->Version.v1.u64BackingFileOffset + pHdr->Version.v1.u32BackingFileSize > cbFile)
            return vdIfError(pImage->pIfError, VERR_INVALID_STATE, RT_SRC_POS,
                             N_("QCOW: Backing file offset and size exceed size of image '%s' (%u vs %u)"),
                             pImage->pszFilename, pHdr->Version.v1.u64BackingFileOffset + pHdr->Version.v1.u32BackingFileSize,
                             cbFile);

        /* Check that the cluster bits indicate at least a 512byte sector size. */
        if (RT_BIT_32(pHdr->Version.v1.u8ClusterBits) < 512)
            return vdIfError(pImage->pIfError, VERR_INVALID_STATE, RT_SRC_POS,
                             N_("QCOW: Cluster size is too small for image  '%s' (%u vs %u)"),
                             pImage->pszFilename, RT_BIT_32(pHdr->Version.v1.u8ClusterBits), 512);

        /*
         * Check for possible overflow when multiplying cluster size and L2 entry count because it is used
         * to calculate the number of L1 table entries later on.
         */
        if (RT_BIT_32(pHdr->Version.v1.u8L2Bits) * RT_BIT_32(pHdr->Version.v1.u8ClusterBits) == 0)
            return vdIfError(pImage->pIfError, VERR_INVALID_STATE, RT_SRC_POS,
                             N_("QCOW: Overflow during L1 table size calculation for image '%s'"),
                             pImage->pszFilename);
    }
    else if (pHdr->u32Version == 2 || pHdr->u32Version == 3)
    {
        /* Check that the backing filename is contained in the file. */
        if (pHdr->Version.v2.u64BackingFileOffset + pHdr->Version.v2.u32BackingFileSize > cbFile)
            return vdIfError(pImage->pIfError, VERR_INVALID_STATE, RT_SRC_POS,
                             N_("QCOW: Backing file offset and size exceed size of image '%s' (%u vs %u)"),
                             pImage->pszFilename, pHdr->Version.v2.u64BackingFileOffset + pHdr->Version.v2.u32BackingFileSize,
                             cbFile);

        /* Check that the cluster bits indicate at least a 512byte sector size. */
        if (RT_BIT_32(pHdr->Version.v2.u32ClusterBits) < 512)
            return vdIfError(pImage->pIfError, VERR_INVALID_STATE, RT_SRC_POS,
                             N_("QCOW: Cluster size is too small for image  '%s' (%u vs %u)"),
                             pImage->pszFilename, RT_BIT_32(pHdr->Version.v2.u32ClusterBits), 512);

        /* Some additional checks for v3 images. */
        if (pHdr->u32Version == 3)
        {
            if (pHdr->Version.v2.v3.u32RefCntWidth > 6)
                return vdIfError(pImage->pIfError, VERR_INVALID_STATE, RT_SRC_POS,
                                 N_("QCOW: Reference count width too big for image  '%s' (%u vs %u)"),
                                 pImage->pszFilename, RT_BIT_32(pHdr->Version.v2.v3.u32RefCntWidth), 6);
        }
    }
    else
        return vdIfError(pImage->pIfError, VERR_NOT_SUPPORTED, RT_SRC_POS,
                         N_("QCOW: Version %u in image '%s' is not supported"),
                         pHdr->u32Version, pImage->pszFilename);

    return VINF_SUCCESS;
}

/**
 * Internal: Open an image, constructing all necessary data structures.
 */
static int qcowOpenImage(PQCOWIMAGE pImage, unsigned uOpenFlags)
{
    pImage->uOpenFlags = uOpenFlags;

    pImage->pIfError = VDIfErrorGet(pImage->pVDIfsDisk);
    pImage->pIfIo = VDIfIoIntGet(pImage->pVDIfsImage);
    AssertPtrReturn(pImage->pIfIo, VERR_INVALID_PARAMETER);

    int rc = qcowL2TblCacheCreate(pImage);
    if (RT_SUCCESS(rc))
    {
        /* Open the image. */
        rc = vdIfIoIntFileOpen(pImage->pIfIo, pImage->pszFilename,
                               VDOpenFlagsToFileOpenFlags(uOpenFlags,
                                                          false /* fCreate */),
                               &pImage->pStorage);
        if (RT_SUCCESS(rc))
        {
            uint64_t cbFile;
            rc = vdIfIoIntFileGetSize(pImage->pIfIo, pImage->pStorage, &cbFile);
            if (   RT_SUCCESS(rc)
                && cbFile > sizeof(QCowHeader))
            {
                QCowHeader Header;

                rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage, 0, &Header, sizeof(Header));
                if (   RT_SUCCESS(rc)
                    && qcowHdrConvertToHostEndianess(&Header))
                {
                    pImage->offNextCluster = RT_ALIGN_64(cbFile, 512); /* Align image to sector boundary. */
                    Assert(pImage->offNextCluster >= cbFile);

                    rc = qcowHdrValidate(pImage, &Header, cbFile);
                    if (RT_SUCCESS(rc))
                    {
                        if (Header.u32Version == 1)
                        {
                            if (!Header.Version.v1.u32CryptMethod)
                            {
                                pImage->uVersion           = 1;
                                pImage->offBackingFilename = Header.Version.v1.u64BackingFileOffset;
                                pImage->cbBackingFilename  = Header.Version.v1.u32BackingFileSize;
                                pImage->MTime              = Header.Version.v1.u32MTime;
                                pImage->cbSize             = Header.Version.v1.u64Size;
                                pImage->cClusterBits       = Header.Version.v1.u8ClusterBits;
                                pImage->cbCluster          = RT_BIT_32(Header.Version.v1.u8ClusterBits);
                                pImage->cL2TableEntries    = RT_BIT_32(Header.Version.v1.u8L2Bits);
                                pImage->cbL2Table          = RT_ALIGN_64(pImage->cL2TableEntries * sizeof(uint64_t), pImage->cbCluster);
                                pImage->offL1Table         = Header.Version.v1.u64L1TableOffset;
                                pImage->cL1TableEntries    = pImage->cbSize / (pImage->cbCluster * pImage->cL2TableEntries);
                                if (pImage->cbSize % (pImage->cbCluster * pImage->cL2TableEntries))
                                    pImage->cL1TableEntries++;
                            }
                            else
                                rc = vdIfError(pImage->pIfError, VERR_NOT_SUPPORTED, RT_SRC_POS,
                                               N_("QCow: Encrypted image '%s' is not supported"),
                                               pImage->pszFilename);
                        }
                        else if (Header.u32Version == 2 || Header.u32Version == 3)
                        {
                            if (Header.Version.v2.u32CryptMethod)
                                rc = vdIfError(pImage->pIfError, VERR_NOT_SUPPORTED, RT_SRC_POS,
                                               N_("QCow: Encrypted image '%s' is not supported"),
                                               pImage->pszFilename);
                            else if (Header.Version.v2.u32NbSnapshots)
                                rc = vdIfError(pImage->pIfError, VERR_NOT_SUPPORTED, RT_SRC_POS,
                                               N_("QCow: Image '%s' contains snapshots which is not supported"),
                                               pImage->pszFilename);
                            else
                            {
                                pImage->uVersion              = 2;
                                pImage->offBackingFilename    = Header.Version.v2.u64BackingFileOffset;
                                pImage->cbBackingFilename     = Header.Version.v2.u32BackingFileSize;
                                pImage->cbSize                = Header.Version.v2.u64Size;
                                pImage->cClusterBits          = Header.Version.v2.u32ClusterBits;
                                pImage->cbCluster             = RT_BIT_32(Header.Version.v2.u32ClusterBits);
                                pImage->cL2TableEntries       = pImage->cbCluster / sizeof(uint64_t);
                                pImage->cbL2Table             = pImage->cbCluster;
                                pImage->offL1Table            = Header.Version.v2.u64L1TableOffset;
                                pImage->cL1TableEntries       = Header.Version.v2.u32L1Size;
                                pImage->offRefcountTable      = Header.Version.v2.u64RefcountTableOffset;
                                pImage->cbRefcountTable       = qcowCluster2Byte(pImage, Header.Version.v2.u32RefcountTableClusters);
                                pImage->cRefcountTableEntries = pImage->cbRefcountTable / sizeof(uint64_t);

                                /* Init the masks to extract offset and sector count from a compressed cluster descriptor. */
                                uint32_t cBitsCompressedClusterOffset = 62 - (pImage->cClusterBits - 8);
                                pImage->fMaskCompressedClusterOffset  = RT_BIT_64(cBitsCompressedClusterOffset) - 1;
                                pImage->fMaskCompressedClusterSectors = (RT_BIT_64(62) - 1) & ~pImage->fMaskCompressedClusterOffset;
                                pImage->cBitsShiftRCompressedClusterSectors = cBitsCompressedClusterOffset;

                                if (Header.u32Version == 3)
                                {
                                    if (Header.Version.v2.v3.u64IncompatFeat & ~QCOW_V3_INCOMPAT_FEAT_SUPPORTED_MASK)
                                        rc = vdIfError(pImage->pIfError, VERR_NOT_SUPPORTED, RT_SRC_POS,
                                                       N_("QCow: Image '%s' contains unsupported incompatible features (%llx vs %llx)"),
                                                       pImage->pszFilename, Header.Version.v2.v3.u64IncompatFeat, QCOW_V3_INCOMPAT_FEAT_SUPPORTED_MASK);

                                    /** @todo Auto clear features need to be reset as soon as write support is added. */
                                }
                            }
                        }
                        else
                            rc = vdIfError(pImage->pIfError, VERR_NOT_SUPPORTED, RT_SRC_POS,
                                           N_("QCow: Image '%s' uses version %u which is not supported"),
                                           pImage->pszFilename, Header.u32Version);

                        if (RT_SUCCESS(rc))
                        {
                            pImage->cbL1Table = RT_ALIGN_64(pImage->cL1TableEntries * sizeof(uint64_t), pImage->cbCluster);
                            if ((uint64_t)pImage->cbL1Table != RT_ALIGN_64(pImage->cL1TableEntries * sizeof(uint64_t), pImage->cbCluster))
                                rc = vdIfError(pImage->pIfError, VERR_INVALID_STATE, RT_SRC_POS,
                                               N_("QCOW: L1 table size overflow in image '%s'"),
                                               pImage->pszFilename);
                        }
                    }

                    /** @todo Check that there are no compressed clusters in the image
                     *  (by traversing the L2 tables and checking each offset).
                     *  Refuse to open such images.
                     */

                    if (   RT_SUCCESS(rc)
                        && pImage->cbBackingFilename
                        && pImage->offBackingFilename)
                    {
                        /* Load backing filename from image. */
                        pImage->pszBackingFilename = RTStrAlloc(pImage->cbBackingFilename + 1); /* +1 for \0 terminator. */
                        if (pImage->pszBackingFilename)
                        {
                            RT_BZERO(pImage->pszBackingFilename, pImage->cbBackingFilename + 1);
                            rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
                                                       pImage->offBackingFilename, pImage->pszBackingFilename,
                                                       pImage->cbBackingFilename);
                            if (RT_SUCCESS(rc))
                                rc = RTStrValidateEncoding(pImage->pszBackingFilename);
                        }
                        else
                            rc = VERR_NO_STR_MEMORY;
                    }

                    if (   RT_SUCCESS(rc)
                        && pImage->cbRefcountTable
                        && pImage->offRefcountTable)
                    {
                        /* Load refcount table. */
                        Assert(pImage->cRefcountTableEntries);
                        pImage->paRefcountTable = (uint64_t *)RTMemAllocZ(pImage->cbRefcountTable);
                        if (RT_LIKELY(pImage->paRefcountTable))
                        {
                            rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
                                                       pImage->offRefcountTable, pImage->paRefcountTable,
                                                       pImage->cbRefcountTable);
                            if (RT_SUCCESS(rc))
                                qcowTableConvertToHostEndianess(pImage->paRefcountTable,
                                                                pImage->cRefcountTableEntries);
                            else
                                rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS,
                                               N_("QCow: Reading refcount table of image '%s' failed"),
                                               pImage->pszFilename);
                        }
                        else
                            rc = vdIfError(pImage->pIfError, VERR_NO_MEMORY, RT_SRC_POS,
                                           N_("QCow: Allocating memory for refcount table of image '%s' failed"),
                                           pImage->pszFilename);
                    }

                    if (RT_SUCCESS(rc))
                    {
                        qcowTableMasksInit(pImage);

                        /* Allocate L1 table. */
                        pImage->paL1Table = (uint64_t *)RTMemAllocZ(pImage->cbL1Table);
                        if (pImage->paL1Table)
                        {
                            /* Read from the image. */
                            rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
                                                       pImage->offL1Table, pImage->paL1Table,
                                                       pImage->cbL1Table);
                            if (RT_SUCCESS(rc))
                                qcowTableConvertToHostEndianess(pImage->paL1Table, pImage->cL1TableEntries);
                            else
                                rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS,
                                               N_("QCow: Reading the L1 table for image '%s' failed"),
                                               pImage->pszFilename);
                        }
                        else
                            rc = vdIfError(pImage->pIfError, VERR_NO_MEMORY, RT_SRC_POS,
                                           N_("QCow: Out of memory allocating L1 table for image '%s'"),
                                           pImage->pszFilename);
                    }
                }
                else if (RT_SUCCESS(rc))
                    rc = VERR_VD_GEN_INVALID_HEADER;
            }
            else if (RT_SUCCESS(rc))
                rc = VERR_VD_GEN_INVALID_HEADER;
        }
        /* else: Do NOT signal an appropriate error here, as the VD layer has the
         *       choice of retrying the open if it failed. */
    }
    else
        rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS,
                       N_("Qcow: Creating the L2 table cache for image '%s' failed"),
                       pImage->pszFilename);

    if (RT_SUCCESS(rc))
    {
        PVDREGIONDESC pRegion = &pImage->RegionList.aRegions[0];
        pImage->RegionList.fFlags   = 0;
        pImage->RegionList.cRegions = 1;

        pRegion->offRegion            = 0; /* Disk start. */
        pRegion->cbBlock              = 512;
        pRegion->enmDataForm          = VDREGIONDATAFORM_RAW;
        pRegion->enmMetadataForm      = VDREGIONMETADATAFORM_NONE;
        pRegion->cbData               = 512;
        pRegion->cbMetadata           = 0;
        pRegion->cRegionBlocksOrBytes = pImage->cbSize;
    }
    else
        qcowFreeImage(pImage, false);
    return rc;
}

/**
 * Internal: Create a qcow image.
 */
static int qcowCreateImage(PQCOWIMAGE pImage, uint64_t cbSize,
                           unsigned uImageFlags, const char *pszComment,
                           PCVDGEOMETRY pPCHSGeometry,
                           PCVDGEOMETRY pLCHSGeometry, unsigned uOpenFlags,
                           PVDINTERFACEPROGRESS pIfProgress,
                           unsigned uPercentStart, unsigned uPercentSpan)
{
    RT_NOREF1(pszComment);
    int rc;
    int32_t fOpen;

    if (!(uImageFlags & VD_IMAGE_FLAGS_FIXED))
    {
        rc = qcowL2TblCacheCreate(pImage);
        if (RT_SUCCESS(rc))
        {
            pImage->uOpenFlags   = uOpenFlags & ~VD_OPEN_FLAGS_READONLY;
            pImage->uImageFlags  = uImageFlags;
            pImage->PCHSGeometry = *pPCHSGeometry;
            pImage->LCHSGeometry = *pLCHSGeometry;
            pImage->pIfError = VDIfErrorGet(pImage->pVDIfsDisk);
            pImage->pIfIo = VDIfIoIntGet(pImage->pVDIfsImage);
            AssertPtrReturn(pImage->pIfIo, VERR_INVALID_PARAMETER);

            /* Create image file. */
            fOpen = VDOpenFlagsToFileOpenFlags(pImage->uOpenFlags, true /* fCreate */);
            rc = vdIfIoIntFileOpen(pImage->pIfIo, pImage->pszFilename, fOpen, &pImage->pStorage);
            if (RT_SUCCESS(rc))
            {
                /* Init image state. */
                pImage->uVersion           = 1; /* We create only version 1 images at the moment. */
                pImage->cbSize             = cbSize;
                pImage->cbCluster          = QCOW_CLUSTER_SIZE_DEFAULT;
                pImage->cbL2Table          = qcowCluster2Byte(pImage, QCOW_L2_CLUSTERS_DEFAULT);
                pImage->cL2TableEntries    = pImage->cbL2Table / sizeof(uint64_t);
                pImage->cL1TableEntries    = cbSize / (pImage->cbCluster * pImage->cL2TableEntries);
                if (cbSize % (pImage->cbCluster * pImage->cL2TableEntries))
                    pImage->cL1TableEntries++;
                pImage->cbL1Table          = RT_ALIGN_64(pImage->cL1TableEntries * sizeof(uint64_t), pImage->cbCluster);
                pImage->offL1Table         = QCOW_V1_HDR_SIZE;
                pImage->cbBackingFilename  = 0;
                pImage->offBackingFilename = 0;
                pImage->offNextCluster     = RT_ALIGN_64(QCOW_V1_HDR_SIZE + pImage->cbL1Table, pImage->cbCluster);
                qcowTableMasksInit(pImage);

                /* Init L1 table. */
                pImage->paL1Table = (uint64_t *)RTMemAllocZ(pImage->cbL1Table);
                if (RT_LIKELY(pImage->paL1Table))
                {
                    if (RT_SUCCESS(rc))
                        vdIfProgress(pIfProgress, uPercentStart + uPercentSpan * 98 / 100);

                    rc = qcowFlushImage(pImage);
                    if (RT_SUCCESS(rc))
                        rc = vdIfIoIntFileSetSize(pImage->pIfIo, pImage->pStorage, pImage->offNextCluster);
                }
                else
                    rc = vdIfError(pImage->pIfError, VERR_NO_MEMORY, RT_SRC_POS, N_("QCow: cannot allocate memory for L1 table of image '%s'"),
                                   pImage->pszFilename);
            }
            else
                rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("QCow: cannot create image '%s'"), pImage->pszFilename);
        }
        else
            rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("QCow: Failed to create L2 cache for image '%s'"),
                           pImage->pszFilename);
    }
    else
        rc = vdIfError(pImage->pIfError, VERR_VD_INVALID_TYPE, RT_SRC_POS, N_("QCow: cannot create fixed image '%s'"), pImage->pszFilename);

    if (RT_SUCCESS(rc))
        vdIfProgress(pIfProgress, uPercentStart + uPercentSpan);

    if (RT_SUCCESS(rc))
    {
        PVDREGIONDESC pRegion = &pImage->RegionList.aRegions[0];
        pImage->RegionList.fFlags   = 0;
        pImage->RegionList.cRegions = 1;

        pRegion->offRegion            = 0; /* Disk start. */
        pRegion->cbBlock              = 512;
        pRegion->enmDataForm          = VDREGIONDATAFORM_RAW;
        pRegion->enmMetadataForm      = VDREGIONMETADATAFORM_NONE;
        pRegion->cbData               = 512;
        pRegion->cbMetadata           = 0;
        pRegion->cRegionBlocksOrBytes = pImage->cbSize;
    }
    else
        qcowFreeImage(pImage, rc != VERR_ALREADY_EXISTS);
    return rc;
}

/**
 * Rollback anything done during async cluster allocation.
 *
 * @returns VBox status code.
 * @param   pImage           The image instance data.
 * @param   pIoCtx           The I/O context.
 * @param   pClusterAlloc    The cluster allocation to rollback.
 */
static int qcowAsyncClusterAllocRollback(PQCOWIMAGE pImage, PVDIOCTX pIoCtx, PQCOWCLUSTERASYNCALLOC pClusterAlloc)
{
    RT_NOREF1(pIoCtx);
    int rc = VINF_SUCCESS;

    switch (pClusterAlloc->enmAllocState)
    {
        case QCOWCLUSTERASYNCALLOCSTATE_L2_ALLOC:
        case QCOWCLUSTERASYNCALLOCSTATE_L2_LINK:
        {
            /* Revert the L1 table entry */
            pImage->paL1Table[pClusterAlloc->idxL1] = 0;
            pImage->pL2TblAlloc = NULL;

            /* Assumption right now is that the L1 table is not modified on storage if the link fails. */
            rc = vdIfIoIntFileSetSize(pImage->pIfIo, pImage->pStorage, pClusterAlloc->offNextClusterOld);
            qcowL2TblCacheEntryRelease(pClusterAlloc->pL2Entry); /* Release L2 cache entry. */
            Assert(!pClusterAlloc->pL2Entry->cRefs);
            qcowL2TblCacheEntryFree(pImage, pClusterAlloc->pL2Entry); /* Free it, it is not in the cache yet. */
            break;
        }
        case QCOWCLUSTERASYNCALLOCSTATE_USER_ALLOC:
        case QCOWCLUSTERASYNCALLOCSTATE_USER_LINK:
        {
            /* Assumption right now is that the L2 table is not modified if the link fails. */
            pClusterAlloc->pL2Entry->paL2Tbl[pClusterAlloc->idxL2] = 0;
            rc = vdIfIoIntFileSetSize(pImage->pIfIo, pImage->pStorage, pClusterAlloc->offNextClusterOld);
            qcowL2TblCacheEntryRelease(pClusterAlloc->pL2Entry); /* Release L2 cache entry. */
            break;
        }
        default:
            AssertMsgFailed(("Invalid cluster allocation state %d\n", pClusterAlloc->enmAllocState));
            rc = VERR_INVALID_STATE;
    }

    RTMemFree(pClusterAlloc);
    return rc;
}

/**
 * Updates the state of the async cluster allocation.
 *
 * @returns VBox status code.
 * @param   pBackendData    The opaque backend data.
 * @param   pIoCtx          I/O context associated with this request.
 * @param   pvUser          Opaque user data passed during a read/write request.
 * @param   rcReq           Status code for the completed request.
 */
static DECLCALLBACK(int) qcowAsyncClusterAllocUpdate(void *pBackendData, PVDIOCTX pIoCtx, void *pvUser, int rcReq)
{
    int rc = VINF_SUCCESS;
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;
    PQCOWCLUSTERASYNCALLOC pClusterAlloc = (PQCOWCLUSTERASYNCALLOC)pvUser;

    if (RT_FAILURE(rcReq))
        return qcowAsyncClusterAllocRollback(pImage, pIoCtx, pClusterAlloc);

    AssertPtr(pClusterAlloc->pL2Entry);

    switch (pClusterAlloc->enmAllocState)
    {
        case QCOWCLUSTERASYNCALLOCSTATE_L2_ALLOC:
        {
            /* Update the link in the in memory L1 table now. */
            pImage->paL1Table[pClusterAlloc->idxL1] = pClusterAlloc->pL2Entry->offL2Tbl;

            /* Update the link in the on disk L1 table now. */
            pClusterAlloc->enmAllocState = QCOWCLUSTERASYNCALLOCSTATE_L2_LINK;
            rc = qcowTblWrite(pImage, pIoCtx, pImage->offL1Table, pImage->paL1Table,
                              pImage->cbL1Table, pImage->cL1TableEntries,
                              qcowAsyncClusterAllocUpdate, pClusterAlloc);
            if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                break;
            else if (RT_FAILURE(rc))
            {
                /* Rollback. */
                qcowAsyncClusterAllocRollback(pImage, pIoCtx, pClusterAlloc);
                break;
            }
            /* Success, fall through. */
        }
        RT_FALL_THRU();
        case QCOWCLUSTERASYNCALLOCSTATE_L2_LINK:
        {
            /* L2 link updated in L1 , save L2 entry in cache and allocate new user data cluster. */
            uint64_t offData = qcowClusterAllocate(pImage, 1);

            pImage->pL2TblAlloc = NULL;
            qcowL2TblCacheEntryInsert(pImage, pClusterAlloc->pL2Entry);

            pClusterAlloc->enmAllocState     = QCOWCLUSTERASYNCALLOCSTATE_USER_ALLOC;
            pClusterAlloc->offNextClusterOld = offData;
            pClusterAlloc->offClusterNew     = offData;

            /* Write data. */
            rc = vdIfIoIntFileWriteUser(pImage->pIfIo, pImage->pStorage,
                                        offData, pIoCtx, pClusterAlloc->cbToWrite,
                                        qcowAsyncClusterAllocUpdate, pClusterAlloc);
            if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                break;
            else if (RT_FAILURE(rc))
            {
                qcowAsyncClusterAllocRollback(pImage, pIoCtx, pClusterAlloc);
                RTMemFree(pClusterAlloc);
                break;
            }
        }
        RT_FALL_THRU();
        case QCOWCLUSTERASYNCALLOCSTATE_USER_ALLOC:
        {
            pClusterAlloc->enmAllocState = QCOWCLUSTERASYNCALLOCSTATE_USER_LINK;
            pClusterAlloc->pL2Entry->paL2Tbl[pClusterAlloc->idxL2] = pClusterAlloc->offClusterNew;

            /* Link L2 table and update it. */
            rc = qcowTblWrite(pImage, pIoCtx, pImage->paL1Table[pClusterAlloc->idxL1],
                              pClusterAlloc->pL2Entry->paL2Tbl,
                              pImage->cbL2Table, pImage->cL2TableEntries,
                              qcowAsyncClusterAllocUpdate, pClusterAlloc);
            if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                break;
            else if (RT_FAILURE(rc))
            {
                qcowAsyncClusterAllocRollback(pImage, pIoCtx, pClusterAlloc);
                RTMemFree(pClusterAlloc);
                break;
            }
        }
        RT_FALL_THRU();
        case QCOWCLUSTERASYNCALLOCSTATE_USER_LINK:
        {
            /* Everything done without errors, signal completion. */
            qcowL2TblCacheEntryRelease(pClusterAlloc->pL2Entry);
            RTMemFree(pClusterAlloc);
            rc = VINF_SUCCESS;
            break;
        }
        default:
            AssertMsgFailed(("Invalid async cluster allocation state %d\n",
                             pClusterAlloc->enmAllocState));
    }

    return rc;
}

/**
 * Reads a compressed cluster, inflates it and copies the amount of data requested
 * into the given I/O context.
 *
 * @returns VBox status code.
 * @param   pImage              The image instance data.
 * @param   pIoCtx              The I/O context.
 * @param   offCluster          Where to start reading in the uncompressed cluster.
 * @param   cbToRead            How much to read in the uncomrpessed cluster.
 * @param   offFile             Offset where the compressed cluster is stored in the image.
 * @param   cbCompressedCluster Size of the comrpessed cluster in bytes.
 */
static int qcowReadCompressedCluster(PQCOWIMAGE pImage, PVDIOCTX pIoCtx,
                                     uint32_t offCluster, size_t cbToRead,
                                     uint64_t offFile, size_t cbCompressedCluster)
{
    int rc = VINF_SUCCESS;

    AssertReturn(!(pImage->uOpenFlags & VD_OPEN_FLAGS_ASYNC_IO), VERR_NOT_SUPPORTED); /* Only synchronous I/O supported so far. */

    if (cbCompressedCluster > pImage->cbCompCluster)
    {
        void *pvCompClusterNew = RTMemRealloc(pImage->pvCompCluster, cbCompressedCluster);
        if (RT_LIKELY(pvCompClusterNew))
        {
            pImage->pvCompCluster = pvCompClusterNew;
            pImage->cbCompCluster = cbCompressedCluster;
        }
        else
            rc = VERR_NO_MEMORY;
    }

    if (RT_SUCCESS(rc))
    {
        rc = vdIfIoIntFileReadMeta(pImage->pIfIo, pImage->pStorage,
                                   offFile, pImage->pvCompCluster,
                                   cbCompressedCluster, NULL,
                                   NULL, NULL, NULL);
        if (RT_SUCCESS(rc))
        {
            if (!pImage->pvCluster)
            {
                pImage->pvCluster = RTMemAllocZ(pImage->cbCluster);
                if (!pImage->pvCluster)
                    rc = VERR_NO_MEMORY;
            }

            if (RT_SUCCESS(rc))
            {
                size_t cbDecomp = 0;

                rc = RTZipBlockDecompress(RTZIPTYPE_ZLIB_NO_HEADER, 0 /*fFlags*/,
                                          pImage->pvCompCluster, cbCompressedCluster, NULL,
                                          pImage->pvCluster, pImage->cbCluster, &cbDecomp);
                if (RT_SUCCESS(rc))
                {
                    Assert(cbDecomp == pImage->cbCluster);
                    vdIfIoIntIoCtxCopyTo(pImage->pIfIo, pIoCtx,
                                         (uint8_t *)pImage->pvCluster + offCluster,
                                         cbToRead);
                }
            }
        }
    }

    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnProbe */
static DECLCALLBACK(int) qcowProbe(const char *pszFilename, PVDINTERFACE pVDIfsDisk,
                                   PVDINTERFACE pVDIfsImage, VDTYPE enmDesiredType, VDTYPE *penmType)
{
    RT_NOREF(pVDIfsDisk, enmDesiredType);
    LogFlowFunc(("pszFilename=\"%s\" pVDIfsDisk=%#p pVDIfsImage=%#p\n", pszFilename, pVDIfsDisk, pVDIfsImage));
    PVDIOSTORAGE pStorage = NULL;
    uint64_t cbFile;
    int rc = VINF_SUCCESS;

    /* Get I/O interface. */
    PVDINTERFACEIOINT pIfIo = VDIfIoIntGet(pVDIfsImage);
    AssertPtrReturn(pIfIo, VERR_INVALID_PARAMETER);
    AssertPtrReturn(pszFilename, VERR_INVALID_POINTER);
    AssertReturn(*pszFilename != '\0', VERR_INVALID_PARAMETER);

    /*
     * Open the file and read the footer.
     */
    rc = vdIfIoIntFileOpen(pIfIo, pszFilename,
                           VDOpenFlagsToFileOpenFlags(VD_OPEN_FLAGS_READONLY,
                                                      false /* fCreate */),
                           &pStorage);
    if (RT_SUCCESS(rc))
    {
        rc = vdIfIoIntFileGetSize(pIfIo, pStorage, &cbFile);
        if (   RT_SUCCESS(rc)
            && cbFile > sizeof(QCowHeader))
        {
            QCowHeader Header;

            rc = vdIfIoIntFileReadSync(pIfIo, pStorage, 0, &Header, sizeof(Header));
            if (   RT_SUCCESS(rc)
                && qcowHdrConvertToHostEndianess(&Header))
                *penmType = VDTYPE_HDD;
            else
                rc = VERR_VD_GEN_INVALID_HEADER;
        }
        else
            rc = VERR_VD_GEN_INVALID_HEADER;
    }

    if (pStorage)
        vdIfIoIntFileClose(pIfIo, pStorage);

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnOpen */
static DECLCALLBACK(int) qcowOpen(const char *pszFilename, unsigned uOpenFlags,
                                  PVDINTERFACE pVDIfsDisk, PVDINTERFACE pVDIfsImage,
                                  VDTYPE enmType, void **ppBackendData)
{
    RT_NOREF1(enmType); /**< @todo r=klaus make use of the type info. */

    LogFlowFunc(("pszFilename=\"%s\" uOpenFlags=%#x pVDIfsDisk=%#p pVDIfsImage=%#p enmType=%u ppBackendData=%#p\n",
                 pszFilename, uOpenFlags, pVDIfsDisk, pVDIfsImage, enmType, ppBackendData));
    int rc;

    /* Check open flags. All valid flags are supported. */
    AssertReturn(!(uOpenFlags & ~VD_OPEN_FLAGS_MASK), VERR_INVALID_PARAMETER);
    AssertPtrReturn(pszFilename, VERR_INVALID_POINTER);
    AssertReturn(*pszFilename != '\0', VERR_INVALID_PARAMETER);


    PQCOWIMAGE pImage = (PQCOWIMAGE)RTMemAllocZ(RT_UOFFSETOF(QCOWIMAGE, RegionList.aRegions[1]));
    if (RT_LIKELY(pImage))
    {
        pImage->pszFilename = pszFilename;
        pImage->pStorage = NULL;
        pImage->pVDIfsDisk = pVDIfsDisk;
        pImage->pVDIfsImage = pVDIfsImage;

        rc = qcowOpenImage(pImage, uOpenFlags);
        if (RT_SUCCESS(rc))
            *ppBackendData = pImage;
        else
            RTMemFree(pImage);
    }
    else
        rc = VERR_NO_MEMORY;

    LogFlowFunc(("returns %Rrc (pBackendData=%#p)\n", rc, *ppBackendData));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnCreate */
static DECLCALLBACK(int) qcowCreate(const char *pszFilename, uint64_t cbSize,
                                    unsigned uImageFlags, const char *pszComment,
                                    PCVDGEOMETRY pPCHSGeometry, PCVDGEOMETRY pLCHSGeometry,
                                    PCRTUUID pUuid, unsigned uOpenFlags,
                                    unsigned uPercentStart, unsigned uPercentSpan,
                                    PVDINTERFACE pVDIfsDisk, PVDINTERFACE pVDIfsImage,
                                    PVDINTERFACE pVDIfsOperation, VDTYPE enmType,
                                    void **ppBackendData)
{
    RT_NOREF1(pUuid);
    LogFlowFunc(("pszFilename=\"%s\" cbSize=%llu uImageFlags=%#x pszComment=\"%s\" pPCHSGeometry=%#p pLCHSGeometry=%#p Uuid=%RTuuid uOpenFlags=%#x uPercentStart=%u uPercentSpan=%u pVDIfsDisk=%#p pVDIfsImage=%#p pVDIfsOperation=%#p enmType=%u ppBackendData=%#p\n",
                 pszFilename, cbSize, uImageFlags, pszComment, pPCHSGeometry, pLCHSGeometry, pUuid, uOpenFlags, uPercentStart, uPercentSpan, pVDIfsDisk, pVDIfsImage, pVDIfsOperation, enmType, ppBackendData));
    int rc;

    /* Check the VD container type. */
    if (enmType != VDTYPE_HDD)
        return VERR_VD_INVALID_TYPE;

    /* Check open flags. All valid flags are supported. */
    AssertReturn(!(uOpenFlags & ~VD_OPEN_FLAGS_MASK), VERR_INVALID_PARAMETER);
    AssertPtrReturn(pszFilename, VERR_INVALID_POINTER);
    AssertReturn(*pszFilename != '\0', VERR_INVALID_PARAMETER);
    AssertPtrReturn(pPCHSGeometry, VERR_INVALID_POINTER);
    AssertPtrReturn(pLCHSGeometry, VERR_INVALID_POINTER);

    PQCOWIMAGE pImage = (PQCOWIMAGE)RTMemAllocZ(RT_UOFFSETOF(QCOWIMAGE, RegionList.aRegions[1]));
    if (RT_LIKELY(pImage))
    {
        PVDINTERFACEPROGRESS pIfProgress = VDIfProgressGet(pVDIfsOperation);

        pImage->pszFilename = pszFilename;
        pImage->pStorage = NULL;
        pImage->pVDIfsDisk = pVDIfsDisk;
        pImage->pVDIfsImage = pVDIfsImage;

        rc = qcowCreateImage(pImage, cbSize, uImageFlags, pszComment,
                             pPCHSGeometry, pLCHSGeometry, uOpenFlags,
                             pIfProgress, uPercentStart, uPercentSpan);
        if (RT_SUCCESS(rc))
        {
            /* So far the image is opened in read/write mode. Make sure the
             * image is opened in read-only mode if the caller requested that. */
            if (uOpenFlags & VD_OPEN_FLAGS_READONLY)
            {
                qcowFreeImage(pImage, false);
                rc = qcowOpenImage(pImage, uOpenFlags);
            }

            if (RT_SUCCESS(rc))
                *ppBackendData = pImage;
        }

        if (RT_FAILURE(rc))
            RTMemFree(pImage);
    }
    else
        rc = VERR_NO_MEMORY;

    LogFlowFunc(("returns %Rrc (pBackendData=%#p)\n", rc, *ppBackendData));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnRename */
static DECLCALLBACK(int) qcowRename(void *pBackendData, const char *pszFilename)
{
    LogFlowFunc(("pBackendData=%#p pszFilename=%#p\n", pBackendData, pszFilename));
    int rc = VINF_SUCCESS;
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;

    /* Check arguments. */
    AssertReturn((pImage && pszFilename && *pszFilename), VERR_INVALID_PARAMETER);

    /* Close the image. */
    rc = qcowFreeImage(pImage, false);
    if (RT_SUCCESS(rc))
    {
        /* Rename the file. */
        rc = vdIfIoIntFileMove(pImage->pIfIo, pImage->pszFilename, pszFilename, 0);
        if (RT_SUCCESS(rc))
        {
            /* Update pImage with the new information. */
            pImage->pszFilename = pszFilename;

            /* Open the old image with new name. */
            rc = qcowOpenImage(pImage, pImage->uOpenFlags);
        }
        else
        {
            /* The move failed, try to reopen the original image. */
            int rc2 = qcowOpenImage(pImage, pImage->uOpenFlags);
            if (RT_FAILURE(rc2))
                rc = rc2;
        }
    }

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnClose */
static DECLCALLBACK(int) qcowClose(void *pBackendData, bool fDelete)
{
    LogFlowFunc(("pBackendData=%#p fDelete=%d\n", pBackendData, fDelete));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;

    int rc = qcowFreeImage(pImage, fDelete);
    RTMemFree(pImage);

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

static DECLCALLBACK(int) qcowRead(void *pBackendData, uint64_t uOffset, size_t cbToRead,
                                  PVDIOCTX pIoCtx, size_t *pcbActuallyRead)
{
    LogFlowFunc(("pBackendData=%#p uOffset=%llu pIoCtx=%#p cbToRead=%zu pcbActuallyRead=%#p\n",
                 pBackendData, uOffset, pIoCtx, cbToRead, pcbActuallyRead));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;
    uint32_t offCluster = 0;
    uint32_t idxL1      = 0;
    uint32_t idxL2      = 0;
    uint64_t offFile    = 0;
    int rc;

    AssertPtr(pImage);
    Assert(uOffset % 512 == 0);
    Assert(cbToRead % 512 == 0);
    AssertPtrReturn(pIoCtx, VERR_INVALID_POINTER);
    AssertReturn(cbToRead, VERR_INVALID_PARAMETER);
    AssertReturn(uOffset + cbToRead <= pImage->cbSize, VERR_INVALID_PARAMETER);

    qcowConvertLogicalOffset(pImage, uOffset, &idxL1, &idxL2, &offCluster);

    /* Clip read size to remain in the cluster. */
    cbToRead = RT_MIN(cbToRead, pImage->cbCluster - offCluster);

    /* Get offset in image. */
    bool fCompressedCluster = false;
    size_t cbCompressedCluster = 0;
    rc = qcowConvertToImageOffset(pImage, pIoCtx, idxL1, idxL2, offCluster,
                                  &offFile, &fCompressedCluster, &cbCompressedCluster);
    if (RT_SUCCESS(rc))
    {
        if (!fCompressedCluster)
            rc = vdIfIoIntFileReadUser(pImage->pIfIo, pImage->pStorage, offFile,
                                       pIoCtx, cbToRead);
        else
            rc = qcowReadCompressedCluster(pImage, pIoCtx, offCluster, cbToRead, offFile, cbCompressedCluster);
    }

    if (   (   RT_SUCCESS(rc)
            || rc == VERR_VD_BLOCK_FREE
            || rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
        && pcbActuallyRead)
        *pcbActuallyRead = cbToRead;

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

static DECLCALLBACK(int) qcowWrite(void *pBackendData, uint64_t uOffset, size_t cbToWrite,
                                   PVDIOCTX pIoCtx, size_t *pcbWriteProcess, size_t *pcbPreRead,
                                   size_t *pcbPostRead, unsigned fWrite)
{
    LogFlowFunc(("pBackendData=%#p uOffset=%llu pIoCtx=%#p cbToWrite=%zu pcbWriteProcess=%#p pcbPreRead=%#p pcbPostRead=%#p\n",
                 pBackendData, uOffset, pIoCtx, cbToWrite, pcbWriteProcess, pcbPreRead, pcbPostRead));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;
    uint32_t offCluster = 0;
    uint32_t idxL1      = 0;
    uint32_t idxL2      = 0;
    uint64_t offImage   = 0;
    int rc = VINF_SUCCESS;

    AssertPtr(pImage);
    Assert(!(uOffset % 512));
    Assert(!(cbToWrite % 512));
    AssertPtrReturn(pIoCtx, VERR_INVALID_POINTER);
    AssertReturn(cbToWrite, VERR_INVALID_PARAMETER);
    AssertReturn(uOffset + cbToWrite <= pImage->cbSize, VERR_INVALID_PARAMETER);

    if (!(pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY))
    {
        /* Convert offset to L1, L2 index and cluster offset. */
        qcowConvertLogicalOffset(pImage, uOffset, &idxL1, &idxL2, &offCluster);

        /* Clip write size to remain in the cluster. */
        cbToWrite = RT_MIN(cbToWrite, pImage->cbCluster - offCluster);
        Assert(!(cbToWrite % 512));

        /* Get offset in image. */
        bool fCompressedCluster = false;
        size_t cbCompressedCluster = 0;
        rc = qcowConvertToImageOffset(pImage, pIoCtx, idxL1, idxL2, offCluster,
                                      &offImage, &fCompressedCluster, &cbCompressedCluster);
        if (RT_SUCCESS(rc))
        {
            if (!fCompressedCluster)
                rc = vdIfIoIntFileWriteUser(pImage->pIfIo, pImage->pStorage,
                                            offImage, pIoCtx, cbToWrite, NULL, NULL);
            else
                rc = VERR_NOT_SUPPORTED; /** @todo Support writing compressed clusters */
        }
        else if (rc == VERR_VD_BLOCK_FREE)
        {
            if (   cbToWrite == pImage->cbCluster
                && !(fWrite & VD_WRITE_NO_ALLOC))
            {
                PQCOWL2CACHEENTRY pL2Entry = NULL;

                /* Full cluster write to previously unallocated cluster.
                 * Allocate cluster and write data. */
                Assert(!offCluster);

                do
                {
                    /* Check if we have to allocate a new cluster for L2 tables. */
                    if (!pImage->paL1Table[idxL1])
                    {
                        uint64_t offL2Tbl;
                        PQCOWCLUSTERASYNCALLOC pL2ClusterAlloc = NULL;

                        /* Allocate new async cluster allocation state. */
                        pL2ClusterAlloc = (PQCOWCLUSTERASYNCALLOC)RTMemAllocZ(sizeof(QCOWCLUSTERASYNCALLOC));
                        if (RT_UNLIKELY(!pL2ClusterAlloc))
                        {
                            rc = VERR_NO_MEMORY;
                            break;
                        }

                        pL2Entry = qcowL2TblCacheEntryAlloc(pImage);
                        if (!pL2Entry)
                        {
                            rc = VERR_NO_MEMORY;
                            RTMemFree(pL2ClusterAlloc);
                            break;
                        }

                        offL2Tbl = qcowClusterAllocate(pImage, qcowByte2Cluster(pImage, pImage->cbL2Table));
                        pL2Entry->offL2Tbl = offL2Tbl;
                        memset(pL2Entry->paL2Tbl, 0, pImage->cbL2Table);

                        pL2ClusterAlloc->enmAllocState     = QCOWCLUSTERASYNCALLOCSTATE_L2_ALLOC;
                        pL2ClusterAlloc->offNextClusterOld = offL2Tbl;
                        pL2ClusterAlloc->offClusterNew     = offL2Tbl;
                        pL2ClusterAlloc->idxL1             = idxL1;
                        pL2ClusterAlloc->idxL2             = idxL2;
                        pL2ClusterAlloc->cbToWrite         = cbToWrite;
                        pL2ClusterAlloc->pL2Entry          = pL2Entry;

                        pImage->pL2TblAlloc = pL2Entry;

                        LogFlowFunc(("Allocating new L2 table at cluster offset %llu\n", offL2Tbl));

                        /*
                         * Write the L2 table first and link to the L1 table afterwards.
                         * If something unexpected happens the worst case which can happen
                         * is a leak of some clusters.
                         */
                        rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
                                                    offL2Tbl, pL2Entry->paL2Tbl, pImage->cbL2Table, pIoCtx,
                                                    qcowAsyncClusterAllocUpdate, pL2ClusterAlloc);
                        if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                            break;
                        else if (RT_FAILURE(rc))
                        {
                            RTMemFree(pL2ClusterAlloc);
                            qcowL2TblCacheEntryFree(pImage, pL2Entry);
                            break;
                        }

                        rc = qcowAsyncClusterAllocUpdate(pImage, pIoCtx, pL2ClusterAlloc, rc);
                    }
                    else
                    {
                        LogFlowFunc(("Fetching L2 table at cluster offset %llu\n", pImage->paL1Table[idxL1]));

                        rc = qcowL2TblCacheFetch(pImage, pIoCtx, pImage->paL1Table[idxL1],
                                                 &pL2Entry);
                        if (RT_SUCCESS(rc))
                        {
                            PQCOWCLUSTERASYNCALLOC pDataClusterAlloc = NULL;

                            /* Allocate new async cluster allocation state. */
                            pDataClusterAlloc = (PQCOWCLUSTERASYNCALLOC)RTMemAllocZ(sizeof(QCOWCLUSTERASYNCALLOC));
                            if (RT_UNLIKELY(!pDataClusterAlloc))
                            {
                                rc = VERR_NO_MEMORY;
                                break;
                            }

                            /* Allocate new cluster for the data. */
                            uint64_t offData = qcowClusterAllocate(pImage, 1);

                            pDataClusterAlloc->enmAllocState     = QCOWCLUSTERASYNCALLOCSTATE_USER_ALLOC;
                            pDataClusterAlloc->offNextClusterOld = offData;
                            pDataClusterAlloc->offClusterNew     = offData;
                            pDataClusterAlloc->idxL1             = idxL1;
                            pDataClusterAlloc->idxL2             = idxL2;
                            pDataClusterAlloc->cbToWrite         = cbToWrite;
                            pDataClusterAlloc->pL2Entry          = pL2Entry;

                            /* Write data. */
                            rc = vdIfIoIntFileWriteUser(pImage->pIfIo, pImage->pStorage,
                                                        offData, pIoCtx, cbToWrite,
                                                        qcowAsyncClusterAllocUpdate, pDataClusterAlloc);
                            if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                                break;
                            else if (RT_FAILURE(rc))
                            {
                                RTMemFree(pDataClusterAlloc);
                                break;
                            }

                            rc = qcowAsyncClusterAllocUpdate(pImage, pIoCtx, pDataClusterAlloc, rc);
                        }
                    }

                } while (0);

                *pcbPreRead = 0;
                *pcbPostRead = 0;
            }
            else
            {
                /* Trying to do a partial write to an unallocated cluster. Don't do
                 * anything except letting the upper layer know what to do. */
                *pcbPreRead = offCluster;
                *pcbPostRead = pImage->cbCluster - cbToWrite - *pcbPreRead;
            }
        }

        if (pcbWriteProcess)
            *pcbWriteProcess = cbToWrite;
    }
    else
        rc = VERR_VD_IMAGE_READ_ONLY;

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

static DECLCALLBACK(int) qcowFlush(void *pBackendData, PVDIOCTX pIoCtx)
{
    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    AssertPtr(pImage);
    AssertPtrReturn(pIoCtx, VERR_INVALID_PARAMETER);

    if (   pImage->pStorage
        && !(pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY))
    {
        QCowHeader Header;

        rc = qcowTblWrite(pImage, pIoCtx, pImage->offL1Table, pImage->paL1Table,
                          pImage->cbL1Table, pImage->cL1TableEntries, NULL, NULL);
        if (RT_SUCCESS(rc) || rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
        {
            /* Write header. */
            size_t cbHeader = 0;
            qcowHdrConvertFromHostEndianess(pImage, &Header, &cbHeader);
            rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
                                        0, &Header, cbHeader,
                                        pIoCtx, NULL, NULL);
            if (RT_SUCCESS(rc) || rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                rc = vdIfIoIntFileFlush(pImage->pIfIo, pImage->pStorage,
                                        pIoCtx, NULL, NULL);
        }
    }

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnGetVersion */
static DECLCALLBACK(unsigned) qcowGetVersion(void *pBackendData)
{
    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;

    AssertPtrReturn(pImage, 0);

    return pImage->uVersion;
}

/** @copydoc VDIMAGEBACKEND::pfnGetFileSize */
static DECLCALLBACK(uint64_t) qcowGetFileSize(void *pBackendData)
{
    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;
    uint64_t cb = 0;

    AssertPtrReturn(pImage, 0);

    uint64_t cbFile;
    if (pImage->pStorage)
    {
        int rc = vdIfIoIntFileGetSize(pImage->pIfIo, pImage->pStorage, &cbFile);
        if (RT_SUCCESS(rc))
            cb += cbFile;
    }

    LogFlowFunc(("returns %lld\n", cb));
    return cb;
}

/** @copydoc VDIMAGEBACKEND::pfnGetPCHSGeometry */
static DECLCALLBACK(int) qcowGetPCHSGeometry(void *pBackendData, PVDGEOMETRY pPCHSGeometry)
{
    LogFlowFunc(("pBackendData=%#p pPCHSGeometry=%#p\n", pBackendData, pPCHSGeometry));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);

    if (pImage->PCHSGeometry.cCylinders)
        *pPCHSGeometry = pImage->PCHSGeometry;
    else
        rc = VERR_VD_GEOMETRY_NOT_SET;

    LogFlowFunc(("returns %Rrc (PCHS=%u/%u/%u)\n", rc, pPCHSGeometry->cCylinders, pPCHSGeometry->cHeads, pPCHSGeometry->cSectors));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnSetPCHSGeometry */
static DECLCALLBACK(int) qcowSetPCHSGeometry(void *pBackendData, PCVDGEOMETRY pPCHSGeometry)
{
    LogFlowFunc(("pBackendData=%#p pPCHSGeometry=%#p PCHS=%u/%u/%u\n",
                 pBackendData, pPCHSGeometry, pPCHSGeometry->cCylinders, pPCHSGeometry->cHeads, pPCHSGeometry->cSectors));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);

    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
        rc = VERR_VD_IMAGE_READ_ONLY;
    else
        pImage->PCHSGeometry = *pPCHSGeometry;

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnGetLCHSGeometry */
static DECLCALLBACK(int) qcowGetLCHSGeometry(void *pBackendData, PVDGEOMETRY pLCHSGeometry)
{
    LogFlowFunc(("pBackendData=%#p pLCHSGeometry=%#p\n", pBackendData, pLCHSGeometry));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);

    if (pImage->LCHSGeometry.cCylinders)
        *pLCHSGeometry = pImage->LCHSGeometry;
    else
        rc = VERR_VD_GEOMETRY_NOT_SET;

    LogFlowFunc(("returns %Rrc (LCHS=%u/%u/%u)\n", rc, pLCHSGeometry->cCylinders,
                 pLCHSGeometry->cHeads, pLCHSGeometry->cSectors));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnSetLCHSGeometry */
static DECLCALLBACK(int) qcowSetLCHSGeometry(void *pBackendData, PCVDGEOMETRY pLCHSGeometry)
{
    LogFlowFunc(("pBackendData=%#p pLCHSGeometry=%#p LCHS=%u/%u/%u\n", pBackendData,
                 pLCHSGeometry, pLCHSGeometry->cCylinders, pLCHSGeometry->cHeads, pLCHSGeometry->cSectors));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);

    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
        rc = VERR_VD_IMAGE_READ_ONLY;
    else
        pImage->LCHSGeometry = *pLCHSGeometry;

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnQueryRegions */
static DECLCALLBACK(int) qcowQueryRegions(void *pBackendData, PCVDREGIONLIST *ppRegionList)
{
    LogFlowFunc(("pBackendData=%#p ppRegionList=%#p\n", pBackendData, ppRegionList));
    PQCOWIMAGE pThis = (PQCOWIMAGE)pBackendData;

    AssertPtrReturn(pThis, VERR_VD_NOT_OPENED);

    *ppRegionList = &pThis->RegionList;
    LogFlowFunc(("returns %Rrc\n", VINF_SUCCESS));
    return VINF_SUCCESS;
}

/** @copydoc VDIMAGEBACKEND::pfnRegionListRelease */
static DECLCALLBACK(void) qcowRegionListRelease(void *pBackendData, PCVDREGIONLIST pRegionList)
{
    RT_NOREF1(pRegionList);
    LogFlowFunc(("pBackendData=%#p pRegionList=%#p\n", pBackendData, pRegionList));
    PQCOWIMAGE pThis = (PQCOWIMAGE)pBackendData;
    AssertPtr(pThis); RT_NOREF(pThis);

    /* Nothing to do here. */
}

/** @copydoc VDIMAGEBACKEND::pfnGetImageFlags */
static DECLCALLBACK(unsigned) qcowGetImageFlags(void *pBackendData)
{
    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;

    AssertPtrReturn(pImage, 0);

    LogFlowFunc(("returns %#x\n", pImage->uImageFlags));
    return pImage->uImageFlags;
}

/** @copydoc VDIMAGEBACKEND::pfnGetOpenFlags */
static DECLCALLBACK(unsigned) qcowGetOpenFlags(void *pBackendData)
{
    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;

    AssertPtrReturn(pImage, 0);

    LogFlowFunc(("returns %#x\n", pImage->uOpenFlags));
    return pImage->uOpenFlags;
}

/** @copydoc VDIMAGEBACKEND::pfnSetOpenFlags */
static DECLCALLBACK(int) qcowSetOpenFlags(void *pBackendData, unsigned uOpenFlags)
{
    LogFlowFunc(("pBackendData=%#p\n uOpenFlags=%#x", pBackendData, uOpenFlags));
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    /* Image must be opened and the new flags must be valid. */
    if (!pImage || (uOpenFlags & ~(  VD_OPEN_FLAGS_READONLY | VD_OPEN_FLAGS_INFO
                                   | VD_OPEN_FLAGS_ASYNC_IO | VD_OPEN_FLAGS_SHAREABLE
                                   | VD_OPEN_FLAGS_SEQUENTIAL | VD_OPEN_FLAGS_SKIP_CONSISTENCY_CHECKS)))
        rc = VERR_INVALID_PARAMETER;
    else
    {
        /* Implement this operation via reopening the image. */
        rc = qcowFreeImage(pImage, false);
        if (RT_SUCCESS(rc))
            rc = qcowOpenImage(pImage, uOpenFlags);
    }

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnGetComment */
VD_BACKEND_CALLBACK_GET_COMMENT_DEF_NOT_SUPPORTED(qcowGetComment);

/** @copydoc VDIMAGEBACKEND::pfnSetComment */
VD_BACKEND_CALLBACK_SET_COMMENT_DEF_NOT_SUPPORTED(qcowSetComment, PQCOWIMAGE);

/** @copydoc VDIMAGEBACKEND::pfnGetUuid */
VD_BACKEND_CALLBACK_GET_UUID_DEF_NOT_SUPPORTED(qcowGetUuid);

/** @copydoc VDIMAGEBACKEND::pfnSetUuid */
VD_BACKEND_CALLBACK_SET_UUID_DEF_NOT_SUPPORTED(qcowSetUuid, PQCOWIMAGE);

/** @copydoc VDIMAGEBACKEND::pfnGetModificationUuid */
VD_BACKEND_CALLBACK_GET_UUID_DEF_NOT_SUPPORTED(qcowGetModificationUuid);

/** @copydoc VDIMAGEBACKEND::pfnSetModificationUuid */
VD_BACKEND_CALLBACK_SET_UUID_DEF_NOT_SUPPORTED(qcowSetModificationUuid, PQCOWIMAGE);

/** @copydoc VDIMAGEBACKEND::pfnGetParentUuid */
VD_BACKEND_CALLBACK_GET_UUID_DEF_NOT_SUPPORTED(qcowGetParentUuid);

/** @copydoc VDIMAGEBACKEND::pfnSetParentUuid */
VD_BACKEND_CALLBACK_SET_UUID_DEF_NOT_SUPPORTED(qcowSetParentUuid, PQCOWIMAGE);

/** @copydoc VDIMAGEBACKEND::pfnGetParentModificationUuid */
VD_BACKEND_CALLBACK_GET_UUID_DEF_NOT_SUPPORTED(qcowGetParentModificationUuid);

/** @copydoc VDIMAGEBACKEND::pfnSetParentModificationUuid */
VD_BACKEND_CALLBACK_SET_UUID_DEF_NOT_SUPPORTED(qcowSetParentModificationUuid, PQCOWIMAGE);

/** @copydoc VDIMAGEBACKEND::pfnDump */
static DECLCALLBACK(void) qcowDump(void *pBackendData)
{
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;

    AssertPtrReturnVoid(pImage);
    vdIfErrorMessage(pImage->pIfError, "Header: Geometry PCHS=%u/%u/%u LCHS=%u/%u/%u cbSector=%llu\n",
                     pImage->PCHSGeometry.cCylinders, pImage->PCHSGeometry.cHeads, pImage->PCHSGeometry.cSectors,
                     pImage->LCHSGeometry.cCylinders, pImage->LCHSGeometry.cHeads, pImage->LCHSGeometry.cSectors,
                     pImage->cbSize / 512);
}

/** @copydoc VDIMAGEBACKEND::pfnGetParentFilename */
static DECLCALLBACK(int) qcowGetParentFilename(void *pBackendData, char **ppszParentFilename)
{
    int rc = VINF_SUCCESS;
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;

    AssertPtr(pImage);
    if (pImage)
        if (pImage->pszBackingFilename)
            *ppszParentFilename = RTStrDup(pImage->pszBackingFilename);
        else
            rc = VERR_NOT_SUPPORTED;
    else
        rc = VERR_VD_NOT_OPENED;

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnSetParentFilename */
static DECLCALLBACK(int) qcowSetParentFilename(void *pBackendData, const char *pszParentFilename)
{
    int rc = VINF_SUCCESS;
    PQCOWIMAGE pImage = (PQCOWIMAGE)pBackendData;

    AssertPtr(pImage);
    if (pImage)
    {
        if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
            rc = VERR_VD_IMAGE_READ_ONLY;
        else if (   pImage->pszBackingFilename
                 && (strlen(pszParentFilename) > pImage->cbBackingFilename))
            rc = VERR_NOT_SUPPORTED; /* The new filename is longer than the old one. */
        else
        {
            if (pImage->pszBackingFilename)
                RTStrFree(pImage->pszBackingFilename);
            pImage->pszBackingFilename = RTStrDup(pszParentFilename);
            if (!pImage->pszBackingFilename)
                rc = VERR_NO_STR_MEMORY;
            else
            {
                if (!pImage->offBackingFilename)
                {
                    /* Allocate new cluster. */
                    uint64_t offData = qcowClusterAllocate(pImage, 1);

                    Assert((offData & UINT32_MAX) == offData);
                    pImage->offBackingFilename = (uint32_t)offData;
                    pImage->cbBackingFilename  = (uint32_t)strlen(pszParentFilename);
                    rc = vdIfIoIntFileSetSize(pImage->pIfIo, pImage->pStorage,
                                              offData + pImage->cbCluster);
                }

                if (RT_SUCCESS(rc))
                    rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage,
                                                pImage->offBackingFilename,
                                                pImage->pszBackingFilename,
                                                strlen(pImage->pszBackingFilename));
            }
        }
    }
    else
        rc = VERR_VD_NOT_OPENED;

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}



const VDIMAGEBACKEND g_QCowBackend =
{
    /* u32Version */
    VD_IMGBACKEND_VERSION,
    /* pszBackendName */
    "QCOW",
    /* uBackendCaps */
    VD_CAP_FILE | VD_CAP_VFS | VD_CAP_CREATE_DYNAMIC | VD_CAP_DIFF | VD_CAP_ASYNC,
    /* paFileExtensions */
    s_aQCowFileExtensions,
    /* paConfigInfo */
    NULL,
    /* pfnProbe */
    qcowProbe,
    /* pfnOpen */
    qcowOpen,
    /* pfnCreate */
    qcowCreate,
    /* pfnRename */
    qcowRename,
    /* pfnClose */
    qcowClose,
    /* pfnRead */
    qcowRead,
    /* pfnWrite */
    qcowWrite,
    /* pfnFlush */
    qcowFlush,
    /* pfnDiscard */
    NULL,
    /* pfnGetVersion */
    qcowGetVersion,
    /* pfnGetFileSize */
    qcowGetFileSize,
    /* pfnGetPCHSGeometry */
    qcowGetPCHSGeometry,
    /* pfnSetPCHSGeometry */
    qcowSetPCHSGeometry,
    /* pfnGetLCHSGeometry */
    qcowGetLCHSGeometry,
    /* pfnSetLCHSGeometry */
    qcowSetLCHSGeometry,
    /* pfnQueryRegions */
    qcowQueryRegions,
    /* pfnRegionListRelease */
    qcowRegionListRelease,
    /* pfnGetImageFlags */
    qcowGetImageFlags,
    /* pfnGetOpenFlags */
    qcowGetOpenFlags,
    /* pfnSetOpenFlags */
    qcowSetOpenFlags,
    /* pfnGetComment */
    qcowGetComment,
    /* pfnSetComment */
    qcowSetComment,
    /* pfnGetUuid */
    qcowGetUuid,
    /* pfnSetUuid */
    qcowSetUuid,
    /* pfnGetModificationUuid */
    qcowGetModificationUuid,
    /* pfnSetModificationUuid */
    qcowSetModificationUuid,
    /* pfnGetParentUuid */
    qcowGetParentUuid,
    /* pfnSetParentUuid */
    qcowSetParentUuid,
    /* pfnGetParentModificationUuid */
    qcowGetParentModificationUuid,
    /* pfnSetParentModificationUuid */
    qcowSetParentModificationUuid,
    /* pfnDump */
    qcowDump,
    /* pfnGetTimestamp */
    NULL,
    /* pfnGetParentTimestamp */
    NULL,
    /* pfnSetParentTimestamp */
    NULL,
    /* pfnGetParentFilename */
    qcowGetParentFilename,
    /* pfnSetParentFilename */
    qcowSetParentFilename,
    /* pfnComposeLocation */
    genericFileComposeLocation,
    /* pfnComposeName */
    genericFileComposeName,
    /* pfnCompact */
    NULL,
    /* pfnResize */
    NULL,
    /* pfnRepair */
    NULL,
    /* pfnTraverseMetadata */
    NULL,
    /* u32VersionEnd */
    VD_IMGBACKEND_VERSION
};