path: root/src/VBox/Devices/PC/BIOS/virtio.c

/* $Id: virtio.c $ */
/** @file
 * VirtIO-SCSI host adapter driver to boot from disks.
 */

/*
 * Copyright (C) 2019-2023 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
 */

#include <stdint.h>
#include <string.h>
#include "biosint.h"
#include "ebda.h"
#include "inlines.h"
#include "pciutil.h"
#include "vds.h"
#include "scsi.h"

//#define DEBUG_VIRTIO 1
#if DEBUG_VIRTIO
# define DBG_VIRTIO(...)        BX_INFO(__VA_ARGS__)
#else
# define DBG_VIRTIO(...)
#endif

/* The maximum CDB size. */
#define VIRTIO_SCSI_CDB_SZ      16
/** Maximum sense data to return. */
#define VIRTIO_SCSI_SENSE_SZ    32

#define VIRTIO_SCSI_RING_ELEM    3

/**
 * VirtIO queue descriptor.
 */
typedef struct
{
    /** 64bit guest physical address of the buffer, split into high and low part because we work in real mode. */
    uint32_t        GCPhysBufLow;
    uint32_t        GCPhysBufHigh;
    /** Length of the buffer in bytes. */
    uint32_t        cbBuf;
    /** Flags for the buffer. */
    uint16_t        fFlags;
    /** Next field where the buffer is continued if _NEXT flag is set. */
    uint16_t        idxNext;
} virtio_q_desc_t;

#define VIRTIO_Q_DESC_F_NEXT     0x1
#define VIRTIO_Q_DESC_F_WRITE    0x2
#define VIRTIO_Q_DESC_F_INDIRECT 0x4

/**
 * VirtIO available ring.
 */
typedef struct
{
    /** Flags. */
    volatile uint16_t        fFlags;
    /** Next index to write an available buffer by the driver. */
    volatile uint16_t        idxNextFree;
    /** The ring - we only provide one entry. */
    volatile uint16_t        au16Ring[VIRTIO_SCSI_RING_ELEM];
    /** Used event index. */
    volatile uint16_t        u16EvtUsed;
} virtio_q_avail_t;

/**
 * VirtIO queue used element.
 */
typedef struct
{
    /** Index of the start of the descriptor chain. */
    uint32_t        u32Id;
    /** Number of bytes used in the descriptor chain. */
    uint32_t        cbUsed;
} virtio_q_used_elem_t;

/**
 * VirtIo used ring.
 */
typedef struct
{
    /** Flags. */
    volatile uint16_t    fFlags;
    /** Index where the next entry would be written by the device. */
    volatile uint16_t    idxNextUsed;
    /** The used ring. */
    virtio_q_used_elem_t aRing[VIRTIO_SCSI_RING_ELEM];
} virtio_q_used_t;

/**
 * VirtIO queue structure we are using, needs to be aligned on a 16byte boundary.
 */
typedef struct
{
    /** The descriptor table, using 3 max. */
    virtio_q_desc_t      aDescTbl[3];
    /** Available ring. */
    virtio_q_avail_t     AvailRing;
    /** Used ring. */
    virtio_q_used_t      UsedRing;
    /** The notification offset for the queue. */
    uint32_t             offNotify;
} virtio_q_t;

/**
 * VirtIO SCSI request structure passed in the queue.
 */
typedef struct
{
    /** The LUN to address. */
    uint8_t                 au8Lun[8];
    /** Request ID - split into low and high part. */
    uint32_t                u32IdLow;
    uint32_t                u32IdHigh;
    /** Task attributes. */
    uint8_t                 u8TaskAttr;
    /** Priority. */
    uint8_t                 u8Prio;
    /** CRN value, usually 0. */
    uint8_t                 u8Crn;
    /** The CDB. */
    uint8_t                 abCdb[VIRTIO_SCSI_CDB_SZ];
} virtio_scsi_req_hdr_t;

/**
 * VirtIO SCSI status structure filled by the device.
 */
typedef struct
{
    /** Returned sense length. */
    uint32_t                cbSense;
    /** Residual amount of bytes left. */
    uint32_t                cbResidual;
    /** Status qualifier. */
    uint16_t                u16StatusQual;
    /** Status code. */
    uint8_t                 u8Status;
    /** Response code. */
    uint8_t                 u8Response;
    /** Sense data. */
    uint8_t                 abSense[VIRTIO_SCSI_SENSE_SZ];
} virtio_scsi_req_sts_t;

/**
 * VirtIO config for the different data structures.
 */
typedef struct
{
    /** BAR where to find it. */
    uint8_t         u8Bar;
    /** Padding. */
    uint8_t         abPad[3];
    /** Offset within the bar. */
    uint32_t        u32Offset;
    /** Length of the structure in bytes. */
    uint32_t        u32Length;
} virtio_bar_cfg_t;

/**
 * VirtIO PCI capability structure.
 */
typedef struct
{
    /** Capability typem should always be PCI_CAP_ID_VNDR*/
    uint8_t         u8PciCapId;
    /** Offset where to find the next capability or 0 if last capability. */
    uint8_t         u8PciCapNext;
    /** Size of the capability in bytes. */
    uint8_t         u8PciCapLen;
    /** VirtIO capability type. */
    uint8_t         u8VirtIoCfgType;
    /** BAR where to find it. */
    uint8_t         u8Bar;
    /** Padding. */
    uint8_t         abPad[3];
    /** Offset within the bar. */
    uint32_t        u32Offset;
    /** Length of the structure in bytes. */
    uint32_t        u32Length;
} virtio_pci_cap_t;

/**
 * VirtIO-SCSI controller data.
 */
typedef struct
{
    /** The queue used - must be first for alignment reasons. */
    virtio_q_t       Queue;
    /** The BAR configs read from the PCI configuration space, see VIRTIO_PCI_CAP_*_CFG,
     * only use 4 because VIRTIO_PCI_CAP_PCI_CFG is not part of this. */
    virtio_bar_cfg_t aBarCfgs[4];
    /** The start offset in the PCI configuration space where to find the VIRTIO_PCI_CAP_PCI_CFG
     * capability for the alternate access method to the registers. */
    uint8_t          u8PciCfgOff;
    /** The notification offset multiplier. */
    uint32_t         u32NotifyOffMult;
    /** PCI bus where the device is located. */
    uint8_t          u8Bus;
    /** Device/Function number. */
    uint8_t          u8DevFn;
    /** The current executed command structure. */
    virtio_scsi_req_hdr_t ScsiReqHdr;
    virtio_scsi_req_sts_t ScsiReqSts;
} virtio_t;

/* The VirtIO specific data must fit into 1KB (statically allocated). */
ct_assert(sizeof(virtio_t) <= 1024);

/** PCI configuration fields. */
#define PCI_CONFIG_CAP                  0x34

#define PCI_CAP_ID_VNDR                 0x09

#define VBOX_VIRTIO_NIL_CFG             0xff

#define VIRTIO_PCI_CAP_COMMON_CFG       0x01
#define VIRTIO_PCI_CAP_NOTIFY_CFG       0x02
#define VIRTIO_PCI_CAP_ISR_CFG          0x03
#define VIRTIO_PCI_CAP_DEVICE_CFG       0x04
#define VIRTIO_PCI_CAP_PCI_CFG          0x05

#define RT_BIT_32(bit) ((uint32_t)(1L << (bit)))

#define VIRTIO_COMMON_REG_DEV_FEAT_SLCT 0x00
#define VIRTIO_COMMON_REG_DEV_FEAT      0x04
# define VIRTIO_CMN_REG_DEV_FEAT_SCSI_INOUT 0x01
#define VIRTIO_COMMON_REG_DRV_FEAT_SLCT 0x08
#define VIRTIO_COMMON_REG_DRV_FEAT      0x0c
#define VIRTIO_COMMON_REG_MSIX_CFG      0x10
#define VIRTIO_COMMON_REG_NUM_QUEUES    0x12
#define VIRTIO_COMMON_REG_DEV_STS       0x14
# define VIRTIO_CMN_REG_DEV_STS_F_RST     0x00
# define VIRTIO_CMN_REG_DEV_STS_F_ACK     0x01
# define VIRTIO_CMN_REG_DEV_STS_F_DRV     0x02
# define VIRTIO_CMN_REG_DEV_STS_F_DRV_OK  0x04
# define VIRTIO_CMN_REG_DEV_STS_F_FEAT_OK 0x08
# define VIRTIO_CMN_REG_DEV_STS_F_DEV_RST 0x40
# define VIRTIO_CMN_REG_DEV_STS_F_FAILED  0x80
#define VIRTIO_COMMON_REG_CFG_GEN       0x15

#define VIRTIO_COMMON_REG_Q_SELECT      0x16
#define VIRTIO_COMMON_REG_Q_SIZE        0x18
#define VIRTIO_COMMON_REG_Q_MSIX_VEC    0x1a
#define VIRTIO_COMMON_REG_Q_ENABLE      0x1c
#define VIRTIO_COMMON_REG_Q_NOTIFY_OFF  0x1e
#define VIRTIO_COMMON_REG_Q_DESC        0x20
#define VIRTIO_COMMON_REG_Q_DRIVER      0x28
#define VIRTIO_COMMON_REG_Q_DEVICE      0x30

#define VIRTIO_DEV_CFG_REG_Q_NUM        0x00
#define VIRTIO_DEV_CFG_REG_SEG_MAX      0x04
#define VIRTIO_DEV_CFG_REG_SECT_MAX     0x08
#define VIRTIO_DEV_CFG_REG_CMD_PER_LUN  0x0c
#define VIRTIO_DEV_CFG_REG_EVT_INFO_SZ  0x10
#define VIRTIO_DEV_CFG_REG_SENSE_SZ     0x14
#define VIRTIO_DEV_CFG_REG_CDB_SZ       0x18
#define VIRTIO_DEV_CFG_REG_MAX_CHANNEL  0x1c
#define VIRTIO_DEV_CFG_REG_MAX_TGT      0x1e
#define VIRTIO_DEV_CFG_REG_MAX_LUN      0x20

#define VIRTIO_SCSI_Q_CONTROL           0x00
#define VIRTIO_SCSI_Q_EVENT             0x01
#define VIRTIO_SCSI_Q_REQUEST           0x02

#define VIRTIO_SCSI_STS_RESPONSE_OK     0x00

static void virtio_reg_set_bar_offset_length(virtio_t __far *virtio, uint8_t u8Bar, uint32_t offReg, uint32_t cb)
{
    pci_write_config_byte(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff  +  4, u8Bar);
    pci_write_config_dword(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff +  8, offReg);
    pci_write_config_dword(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff + 12, cb);
}

static void virtio_reg_common_access_prepare(virtio_t __far *virtio, uint16_t offReg, uint32_t cbAcc)
{
    virtio_reg_set_bar_offset_length(virtio,
                                     virtio->aBarCfgs[VIRTIO_PCI_CAP_COMMON_CFG - 1].u8Bar,
                                     virtio->aBarCfgs[VIRTIO_PCI_CAP_COMMON_CFG - 1].u32Offset + offReg,
                                     cbAcc);
}

static void virtio_reg_dev_access_prepare(virtio_t __far *virtio, uint16_t offReg, uint32_t cbAcc)
{
    virtio_reg_set_bar_offset_length(virtio,
                                     virtio->aBarCfgs[VIRTIO_PCI_CAP_DEVICE_CFG - 1].u8Bar,
                                     virtio->aBarCfgs[VIRTIO_PCI_CAP_DEVICE_CFG - 1].u32Offset + offReg,
                                     cbAcc);
}

static void virtio_reg_notify_access_prepare(virtio_t __far *virtio, uint16_t offReg, uint32_t cbAcc)
{
    virtio_reg_set_bar_offset_length(virtio,
                                     virtio->aBarCfgs[VIRTIO_PCI_CAP_NOTIFY_CFG - 1].u8Bar,
                                     virtio->aBarCfgs[VIRTIO_PCI_CAP_NOTIFY_CFG - 1].u32Offset + offReg,
                                     cbAcc);
}

static void virtio_reg_isr_prepare(virtio_t __far *virtio, uint32_t cbAcc)
{
    virtio_reg_set_bar_offset_length(virtio,
                                     virtio->aBarCfgs[VIRTIO_PCI_CAP_ISR_CFG - 1].u8Bar,
                                     virtio->aBarCfgs[VIRTIO_PCI_CAP_ISR_CFG - 1].u32Offset,
                                     cbAcc);
}

static uint8_t virtio_reg_common_read_u8(virtio_t __far *virtio, uint16_t offReg)
{
    virtio_reg_common_access_prepare(virtio, offReg, sizeof(uint8_t));
    return pci_read_config_byte(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff + sizeof(virtio_pci_cap_t));
}

static void virtio_reg_common_write_u8(virtio_t __far *virtio, uint16_t offReg, uint8_t u8Val)
{
    virtio_reg_common_access_prepare(virtio, offReg, sizeof(uint8_t));
    pci_write_config_byte(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff + sizeof(virtio_pci_cap_t), u8Val);
}

static uint16_t virtio_reg_common_read_u16(virtio_t __far *virtio, uint16_t offReg)
{
    virtio_reg_common_access_prepare(virtio, offReg, sizeof(uint16_t));
    return pci_read_config_word(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff + sizeof(virtio_pci_cap_t));
}

static void virtio_reg_common_write_u16(virtio_t __far *virtio, uint16_t offReg, uint16_t u16Val)
{
    virtio_reg_common_access_prepare(virtio, offReg, sizeof(uint16_t));
    pci_write_config_word(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff + sizeof(virtio_pci_cap_t), u16Val);
}

static void virtio_reg_common_write_u32(virtio_t __far *virtio, uint16_t offReg, uint32_t u32Val)
{
    virtio_reg_common_access_prepare(virtio, offReg, sizeof(uint32_t));
    pci_write_config_dword(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff + sizeof(virtio_pci_cap_t), u32Val);
}

static uint32_t virtio_reg_dev_cfg_read_u32(virtio_t __far *virtio, uint16_t offReg)
{
    virtio_reg_dev_access_prepare(virtio, offReg, sizeof(uint32_t));
    return pci_read_config_dword(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff + sizeof(virtio_pci_cap_t));
}

static void virtio_reg_dev_cfg_write_u32(virtio_t __far *virtio, uint16_t offReg, uint32_t u32Val)
{
    virtio_reg_dev_access_prepare(virtio, offReg, sizeof(uint32_t));
    pci_write_config_dword(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff + sizeof(virtio_pci_cap_t), u32Val);
}

static void virtio_reg_notify_write_u16(virtio_t __far *virtio, uint16_t offReg, uint16_t u16Val)
{
    virtio_reg_notify_access_prepare(virtio, offReg, sizeof(uint16_t));
    pci_write_config_word(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff + sizeof(virtio_pci_cap_t), u16Val);
}

static uint8_t virtio_reg_isr_read_u8(virtio_t __far *virtio)
{
    virtio_reg_isr_prepare(virtio, sizeof(uint8_t));
    return pci_read_config_byte(virtio->u8Bus, virtio->u8DevFn, virtio->u8PciCfgOff + sizeof(virtio_pci_cap_t));
}

/**
 * Converts a segment:offset pair into a 32bit physical address.
 */
static uint32_t virtio_addr_to_phys(void __far *ptr)
{
    return ((uint32_t)FP_SEG(ptr) << 4) + FP_OFF(ptr);
}

int virtio_scsi_cmd_data_out(void __far *pvHba, uint8_t idTgt, uint8_t __far *aCDB,
                             uint8_t cbCDB, uint8_t __far *buffer, uint32_t length)
{
    virtio_t __far *virtio = (virtio_t __far *)pvHba;
    uint16_t idxUsedOld = virtio->Queue.UsedRing.idxNextUsed;

    _fmemset(&virtio->ScsiReqHdr, 0, sizeof(virtio->ScsiReqHdr));
    _fmemset(&virtio->ScsiReqSts, 0, sizeof(virtio->ScsiReqSts));

    virtio->ScsiReqHdr.au8Lun[0] = 0x1;
    virtio->ScsiReqHdr.au8Lun[1] = idTgt;
    virtio->ScsiReqHdr.au8Lun[2] = 0;
    virtio->ScsiReqHdr.au8Lun[3] = 0;
    _fmemcpy(&virtio->ScsiReqHdr.abCdb[0], aCDB, cbCDB);

    /* Fill in the descriptors. */
    virtio->Queue.aDescTbl[0].GCPhysBufLow  = virtio_addr_to_phys(&virtio->ScsiReqHdr);
    virtio->Queue.aDescTbl[0].GCPhysBufHigh = 0;
    virtio->Queue.aDescTbl[0].cbBuf         = sizeof(virtio->ScsiReqHdr);
    virtio->Queue.aDescTbl[0].fFlags        = VIRTIO_Q_DESC_F_NEXT;
    virtio->Queue.aDescTbl[0].idxNext       = 1;

    virtio->Queue.aDescTbl[1].GCPhysBufLow  = virtio_addr_to_phys(buffer);
    virtio->Queue.aDescTbl[1].GCPhysBufHigh = 0;
    virtio->Queue.aDescTbl[1].cbBuf         = length;
    virtio->Queue.aDescTbl[1].fFlags        = VIRTIO_Q_DESC_F_NEXT;
    virtio->Queue.aDescTbl[1].idxNext       = 2;

    virtio->Queue.aDescTbl[2].GCPhysBufLow  = virtio_addr_to_phys(&virtio->ScsiReqSts);
    virtio->Queue.aDescTbl[2].GCPhysBufHigh = 0;
    virtio->Queue.aDescTbl[2].cbBuf         = sizeof(virtio->ScsiReqSts);
    virtio->Queue.aDescTbl[2].fFlags        = VIRTIO_Q_DESC_F_WRITE; /* End of chain. */
    virtio->Queue.aDescTbl[2].idxNext       = 0;

    /* Put it into the queue. */
    virtio->Queue.AvailRing.au16Ring[virtio->Queue.AvailRing.idxNextFree % VIRTIO_SCSI_RING_ELEM] = 0;
    virtio->Queue.AvailRing.idxNextFree++;

    /* Notify the device about the new command. */
    DBG_VIRTIO("VirtIO: Submitting new request, Queue.offNotify=0x%x\n", virtio->Queue.offNotify);
    virtio_reg_notify_write_u16(virtio, virtio->Queue.offNotify, VIRTIO_SCSI_Q_REQUEST);

    /* Wait for it to complete. */
    while (idxUsedOld == virtio->Queue.UsedRing.idxNextUsed);

    DBG_VIRTIO("VirtIO: Request complete u8Response=%u\n", virtio->ScsiReqSts.u8Response);

    /* Read ISR register to de-assert the interrupt, don't need to do anything with it. */
    virtio_reg_isr_read_u8(virtio);

    if (virtio->ScsiReqSts.u8Response != VIRTIO_SCSI_STS_RESPONSE_OK)
        return 4;

    return 0;
}

int virtio_scsi_cmd_data_in(void __far *pvHba, uint8_t idTgt, uint8_t __far *aCDB,
                            uint8_t cbCDB, uint8_t __far *buffer, uint32_t length)
{
    virtio_t __far *virtio = (virtio_t __far *)pvHba;
    uint16_t idxUsedOld = virtio->Queue.UsedRing.idxNextUsed;

    _fmemset(&virtio->ScsiReqHdr, 0, sizeof(virtio->ScsiReqHdr));
    _fmemset(&virtio->ScsiReqSts, 0, sizeof(virtio->ScsiReqSts));

    virtio->ScsiReqHdr.au8Lun[0] = 0x1;
    virtio->ScsiReqHdr.au8Lun[1] = idTgt;
    virtio->ScsiReqHdr.au8Lun[2] = 0;
    virtio->ScsiReqHdr.au8Lun[3] = 0;
    _fmemcpy(&virtio->ScsiReqHdr.abCdb[0], aCDB, cbCDB);

    /* Fill in the descriptors. */
    virtio->Queue.aDescTbl[0].GCPhysBufLow  = virtio_addr_to_phys(&virtio->ScsiReqHdr);
    virtio->Queue.aDescTbl[0].GCPhysBufHigh = 0;
    virtio->Queue.aDescTbl[0].cbBuf         = sizeof(virtio->ScsiReqHdr);
    virtio->Queue.aDescTbl[0].fFlags        = VIRTIO_Q_DESC_F_NEXT;
    virtio->Queue.aDescTbl[0].idxNext       = 1;

    /* No data out buffer, the status comes right after this in the next descriptor. */
    virtio->Queue.aDescTbl[1].GCPhysBufLow  = virtio_addr_to_phys(&virtio->ScsiReqSts);
    virtio->Queue.aDescTbl[1].GCPhysBufHigh = 0;
    virtio->Queue.aDescTbl[1].cbBuf         = sizeof(virtio->ScsiReqSts);
    virtio->Queue.aDescTbl[1].fFlags        = VIRTIO_Q_DESC_F_WRITE | VIRTIO_Q_DESC_F_NEXT;
    virtio->Queue.aDescTbl[1].idxNext       = 2;

    virtio->Queue.aDescTbl[2].GCPhysBufLow  = virtio_addr_to_phys(buffer);
    virtio->Queue.aDescTbl[2].GCPhysBufHigh = 0;
    virtio->Queue.aDescTbl[2].cbBuf         = length;
    virtio->Queue.aDescTbl[2].fFlags        = VIRTIO_Q_DESC_F_WRITE; /* End of chain. */
    virtio->Queue.aDescTbl[2].idxNext       = 0;

    /* Put it into the queue, the index is supposed to be free-running and clipped to the ring size
     * internally. The free running index is what the driver sees. */
    virtio->Queue.AvailRing.au16Ring[virtio->Queue.AvailRing.idxNextFree % VIRTIO_SCSI_RING_ELEM] = 0;
    virtio->Queue.AvailRing.idxNextFree++;

    /* Notify the device about the new command. */
    DBG_VIRTIO("VirtIO: Submitting new request, Queue.offNotify=0x%x\n", virtio->Queue.offNotify);
    virtio_reg_notify_write_u16(virtio, virtio->Queue.offNotify, VIRTIO_SCSI_Q_REQUEST);

    /* Wait for it to complete. */
    while (idxUsedOld == virtio->Queue.UsedRing.idxNextUsed);

    DBG_VIRTIO("VirtIO: Request complete u8Response=%u\n", virtio->ScsiReqSts.u8Response);

    /* Read ISR register to de-assert the interrupt, don't need to do anything with it. */
    virtio_reg_isr_read_u8(virtio);

    if (virtio->ScsiReqSts.u8Response != VIRTIO_SCSI_STS_RESPONSE_OK)
        return 4;

    return 0;
}

/**
 * Initializes the VirtIO SCSI HBA and detects attached devices.
 */
static int virtio_scsi_hba_init(virtio_t __far *virtio, uint8_t u8Bus, uint8_t u8DevFn, uint8_t u8PciCapOffVirtIo)
{
    uint8_t u8PciCapOff;
    uint8_t u8DevStat;

    virtio->u8Bus   = u8Bus;
    virtio->u8DevFn = u8DevFn;

    /*
     * Go through the config space again, read the complete config capabilities
     * this time and fill in the data.
     */
    u8PciCapOff = u8PciCapOffVirtIo;
    while (u8PciCapOff != 0)
    {
        uint8_t u8PciCapId = pci_read_config_byte(u8Bus, u8DevFn, u8PciCapOff);
        uint8_t cbPciCap   = pci_read_config_byte(u8Bus, u8DevFn, u8PciCapOff + 2); /* Capability length. */

        DBG_VIRTIO("Capability ID 0x%x at 0x%x\n", u8PciCapId, u8PciCapOff);

        if (   u8PciCapId == PCI_CAP_ID_VNDR
            && cbPciCap >= sizeof(virtio_pci_cap_t))
        {
            /* Read in the config type and see what we got. */
            uint8_t u8PciVirtioCfg = pci_read_config_byte(u8Bus, u8DevFn, u8PciCapOff + 3);

            DBG_VIRTIO("VirtIO: CFG ID 0x%x\n", u8PciVirtioCfg);
            switch (u8PciVirtioCfg)
            {
                case VIRTIO_PCI_CAP_COMMON_CFG:
                case VIRTIO_PCI_CAP_NOTIFY_CFG:
                case VIRTIO_PCI_CAP_ISR_CFG:
                case VIRTIO_PCI_CAP_DEVICE_CFG:
                {
                    virtio_bar_cfg_t __far *pBarCfg = &virtio->aBarCfgs[u8PciVirtioCfg - 1];

                    pBarCfg->u8Bar     = pci_read_config_byte(u8Bus, u8DevFn, u8PciCapOff + 4);
                    pBarCfg->u32Offset = pci_read_config_dword(u8Bus, u8DevFn, u8PciCapOff + 8);
                    pBarCfg->u32Length = pci_read_config_dword(u8Bus, u8DevFn, u8PciCapOff + 12);
                    if (u8PciVirtioCfg == VIRTIO_PCI_CAP_NOTIFY_CFG)
                    {
                        virtio->u32NotifyOffMult = pci_read_config_dword(u8Bus, u8DevFn, u8PciCapOff + 16);
                        DBG_VIRTIO("VirtIO: u32NotifyOffMult 0x%x\n", virtio->u32NotifyOffMult);
                    }
                    break;
                }
                case VIRTIO_PCI_CAP_PCI_CFG:
                    virtio->u8PciCfgOff = u8PciCapOff;
                    DBG_VIRTIO("VirtIO PCI CAP window offset: %x\n", u8PciCapOff);
                    break;
                default:
                    DBG_VIRTIO("VirtIO SCSI HBA with unknown PCI capability type 0x%x\n", u8PciVirtioCfg);
                    break;
            }
        }

        u8PciCapOff = pci_read_config_byte(u8Bus, u8DevFn, u8PciCapOff + 1);
    }

    /* Reset the device. */
    u8DevStat = VIRTIO_CMN_REG_DEV_STS_F_RST;
    virtio_reg_common_write_u8(virtio, VIRTIO_COMMON_REG_DEV_STS, u8DevStat);
    /* Acknowledge presence. */
    u8DevStat |= VIRTIO_CMN_REG_DEV_STS_F_ACK;
    virtio_reg_common_write_u8(virtio, VIRTIO_COMMON_REG_DEV_STS, u8DevStat);
    /* Our driver knows how to operate the device. */
    u8DevStat |= VIRTIO_CMN_REG_DEV_STS_F_DRV;
    virtio_reg_common_write_u8(virtio, VIRTIO_COMMON_REG_DEV_STS, u8DevStat);

#if 0
    /* Read the feature bits and only program the VIRTIO_CMN_REG_DEV_FEAT_SCSI_INOUT bit if available. */
    fFeatures = virtio_reg_common_read_u32(virtio, VIRTIO_COMMON_REG_DEV_FEAT);
    fFeatures &= VIRTIO_CMN_REG_DEV_FEAT_SCSI_INOUT;
#endif

    /* Check that the device is sane. */
    if (   virtio_reg_dev_cfg_read_u32(virtio, VIRTIO_DEV_CFG_REG_Q_NUM) < 1
        || virtio_reg_dev_cfg_read_u32(virtio, VIRTIO_DEV_CFG_REG_CDB_SZ) < 16
        || virtio_reg_dev_cfg_read_u32(virtio, VIRTIO_DEV_CFG_REG_SENSE_SZ) < 32
        || virtio_reg_dev_cfg_read_u32(virtio, VIRTIO_DEV_CFG_REG_SECT_MAX) < 1)
    {
        DBG_VIRTIO("VirtIO-SCSI: Invalid SCSI device configuration, ignoring device\n");
        return 1;
    }

    virtio_reg_common_write_u32(virtio, VIRTIO_COMMON_REG_DRV_FEAT, VIRTIO_CMN_REG_DEV_FEAT_SCSI_INOUT);

    /* Set the features OK bit. */
    u8DevStat |= VIRTIO_CMN_REG_DEV_STS_F_FEAT_OK;
    virtio_reg_common_write_u8(virtio, VIRTIO_COMMON_REG_DEV_STS, u8DevStat);

    /* Read again and check the the okay bit is still set. */
    if (!(virtio_reg_common_read_u8(virtio, VIRTIO_COMMON_REG_DEV_STS) & VIRTIO_CMN_REG_DEV_STS_F_FEAT_OK))
    {
        DBG_VIRTIO("VirtIO-SCSI: Device doesn't accept our feature set, ignoring device\n");
        return 1;
    }

    /* Disable event and control queue. */
    virtio_reg_common_write_u16(virtio, VIRTIO_COMMON_REG_Q_SELECT, VIRTIO_SCSI_Q_CONTROL);
    virtio_reg_common_write_u16(virtio, VIRTIO_COMMON_REG_Q_SIZE, 0);
    virtio_reg_common_write_u16(virtio, VIRTIO_COMMON_REG_Q_ENABLE, 0);

    virtio_reg_common_write_u16(virtio, VIRTIO_COMMON_REG_Q_SELECT, VIRTIO_SCSI_Q_EVENT);
    virtio_reg_common_write_u16(virtio, VIRTIO_COMMON_REG_Q_SIZE, 0);
    virtio_reg_common_write_u16(virtio, VIRTIO_COMMON_REG_Q_ENABLE, 0);

    /* Setup the request queue. */
    virtio_reg_common_write_u16(virtio, VIRTIO_COMMON_REG_Q_SELECT, VIRTIO_SCSI_Q_REQUEST);
    virtio_reg_common_write_u16(virtio, VIRTIO_COMMON_REG_Q_SIZE, VIRTIO_SCSI_RING_ELEM);
    virtio_reg_common_write_u16(virtio, VIRTIO_COMMON_REG_Q_ENABLE, 1);

    /* Set queue area addresses (only low part, leave high part 0). */
    virtio_reg_common_write_u32(virtio, VIRTIO_COMMON_REG_Q_DESC,     virtio_addr_to_phys(&virtio->Queue.aDescTbl[0]));
    virtio_reg_common_write_u32(virtio, VIRTIO_COMMON_REG_Q_DESC + 4, 0);

    virtio_reg_common_write_u32(virtio, VIRTIO_COMMON_REG_Q_DRIVER,     virtio_addr_to_phys(&virtio->Queue.AvailRing));
    virtio_reg_common_write_u32(virtio, VIRTIO_COMMON_REG_Q_DRIVER + 4, 0);

    virtio_reg_common_write_u32(virtio, VIRTIO_COMMON_REG_Q_DEVICE,     virtio_addr_to_phys(&virtio->Queue.UsedRing));
    virtio_reg_common_write_u32(virtio, VIRTIO_COMMON_REG_Q_DEVICE + 4, 0);

    virtio_reg_dev_cfg_write_u32(virtio, VIRTIO_DEV_CFG_REG_CDB_SZ, VIRTIO_SCSI_CDB_SZ);
    virtio_reg_dev_cfg_write_u32(virtio, VIRTIO_DEV_CFG_REG_SENSE_SZ, VIRTIO_SCSI_SENSE_SZ);

    DBG_VIRTIO("VirtIO: Q notify offset 0x%x\n", virtio_reg_common_read_u16(virtio, VIRTIO_COMMON_REG_Q_NOTIFY_OFF));
    virtio->Queue.offNotify = virtio_reg_common_read_u16(virtio, VIRTIO_COMMON_REG_Q_NOTIFY_OFF) * virtio->u32NotifyOffMult;

    /* Bring the device into operational mode. */
    u8DevStat |= VIRTIO_CMN_REG_DEV_STS_F_DRV_OK;
    virtio_reg_common_write_u8(virtio, VIRTIO_COMMON_REG_DEV_STS, u8DevStat);

    return 0;
}

/**
 * Init the VirtIO SCSI driver and detect attached disks.
 */
int virtio_scsi_init(void __far *pvHba, uint8_t u8Bus, uint8_t u8DevFn)
{
    virtio_t __far *virtio = (virtio_t __far *)pvHba;
    uint8_t     u8PciCapOff;
    uint8_t     u8PciCapOffVirtIo = VBOX_VIRTIO_NIL_CFG;
    uint8_t     u8PciCapVirtioSeen = 0;

    /* Examine the capability list and search for the VirtIO specific capabilities. */
    u8PciCapOff = pci_read_config_byte(u8Bus, u8DevFn, PCI_CONFIG_CAP);

    while (u8PciCapOff != 0)
    {
        uint8_t u8PciCapId = pci_read_config_byte(u8Bus, u8DevFn, u8PciCapOff);
        uint8_t cbPciCap   = pci_read_config_byte(u8Bus, u8DevFn, u8PciCapOff + 2); /* Capability length. */

        DBG_VIRTIO("Capability ID 0x%x at 0x%x\n", u8PciCapId, u8PciCapOff);

        if (   u8PciCapId == PCI_CAP_ID_VNDR
            && cbPciCap >= sizeof(virtio_pci_cap_t))
        {
            /* Read in the config type and see what we got. */
            uint8_t u8PciVirtioCfg = pci_read_config_byte(u8Bus, u8DevFn, u8PciCapOff + 3);

            if (u8PciCapOffVirtIo == VBOX_VIRTIO_NIL_CFG)
                u8PciCapOffVirtIo = u8PciCapOff;

            DBG_VIRTIO("VirtIO: CFG ID 0x%x\n", u8PciVirtioCfg);
            switch (u8PciVirtioCfg)
            {
                case VIRTIO_PCI_CAP_COMMON_CFG:
                case VIRTIO_PCI_CAP_NOTIFY_CFG:
                case VIRTIO_PCI_CAP_ISR_CFG:
                case VIRTIO_PCI_CAP_DEVICE_CFG:
                case VIRTIO_PCI_CAP_PCI_CFG:
                    u8PciCapVirtioSeen |= 1 << (u8PciVirtioCfg - 1);
                    break;
                default:
                    DBG_VIRTIO("VirtIO SCSI HBA with unknown PCI capability type 0x%x\n", u8PciVirtioCfg);
            }
        }

        u8PciCapOff = pci_read_config_byte(u8Bus, u8DevFn, u8PciCapOff + 1);
    }

    /* Initialize the controller if all required PCI capabilities where found. */
    if (   u8PciCapOffVirtIo != VBOX_VIRTIO_NIL_CFG
        && u8PciCapVirtioSeen == 0x1f)
    {
        DBG_VIRTIO("VirtIO SCSI HBA with all required capabilities at 0x%x\n", u8PciCapOffVirtIo);

        /* Enable PCI memory, I/O, bus mastering access in command register. */
        pci_write_config_word(u8Bus, u8DevFn, 4, 0x7);
        return virtio_scsi_hba_init(virtio, u8Bus, u8DevFn, u8PciCapOffVirtIo);
    }
    else
        DBG_VIRTIO("VirtIO SCSI HBA with no usable PCI config access!\n");

    return 1;
}