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+=====================
+Booting AArch64 Linux
+=====================
+
+Author: Will Deacon <will.deacon@arm.com>
+
+Date  : 07 September 2012
+
+This document is based on the ARM booting document by Russell King and
+is relevant to all public releases of the AArch64 Linux kernel.
+
+The AArch64 exception model is made up of a number of exception levels
+(EL0 - EL3), with EL0 and EL1 having a secure and a non-secure
+counterpart.  EL2 is the hypervisor level and exists only in non-secure
+mode. EL3 is the highest priority level and exists only in secure mode.
+
+For the purposes of this document, we will use the term `boot loader`
+simply to define all software that executes on the CPU(s) before control
+is passed to the Linux kernel.  This may include secure monitor and
+hypervisor code, or it may just be a handful of instructions for
+preparing a minimal boot environment.
+
+Essentially, the boot loader should provide (as a minimum) the
+following:
+
+1. Setup and initialise the RAM
+2. Setup the device tree
+3. Decompress the kernel image
+4. Call the kernel image
+
+
+1. Setup and initialise RAM
+---------------------------
+
+Requirement: MANDATORY
+
+The boot loader is expected to find and initialise all RAM that the
+kernel will use for volatile data storage in the system.  It performs
+this in a machine dependent manner.  (It may use internal algorithms
+to automatically locate and size all RAM, or it may use knowledge of
+the RAM in the machine, or any other method the boot loader designer
+sees fit.)
+
+
+2. Setup the device tree
+-------------------------
+
+Requirement: MANDATORY
+
+The device tree blob (dtb) must be placed on an 8-byte boundary and must
+not exceed 2 megabytes in size. Since the dtb will be mapped cacheable
+using blocks of up to 2 megabytes in size, it must not be placed within
+any 2M region which must be mapped with any specific attributes.
+
+NOTE: versions prior to v4.2 also require that the DTB be placed within
+the 512 MB region starting at text_offset bytes below the kernel Image.
+
+3. Decompress the kernel image
+------------------------------
+
+Requirement: OPTIONAL
+
+The AArch64 kernel does not currently provide a decompressor and
+therefore requires decompression (gzip etc.) to be performed by the boot
+loader if a compressed Image target (e.g. Image.gz) is used.  For
+bootloaders that do not implement this requirement, the uncompressed
+Image target is available instead.
+
+
+4. Call the kernel image
+------------------------
+
+Requirement: MANDATORY
+
+The decompressed kernel image contains a 64-byte header as follows::
+
+  u32 code0;			/* Executable code */
+  u32 code1;			/* Executable code */
+  u64 text_offset;		/* Image load offset, little endian */
+  u64 image_size;		/* Effective Image size, little endian */
+  u64 flags;			/* kernel flags, little endian */
+  u64 res2	= 0;		/* reserved */
+  u64 res3	= 0;		/* reserved */
+  u64 res4	= 0;		/* reserved */
+  u32 magic	= 0x644d5241;	/* Magic number, little endian, "ARM\x64" */
+  u32 res5;			/* reserved (used for PE COFF offset) */
+
+
+Header notes:
+
+- As of v3.17, all fields are little endian unless stated otherwise.
+
+- code0/code1 are responsible for branching to stext.
+
+- when booting through EFI, code0/code1 are initially skipped.
+  res5 is an offset to the PE header and the PE header has the EFI
+  entry point (efi_stub_entry).  When the stub has done its work, it
+  jumps to code0 to resume the normal boot process.
+
+- Prior to v3.17, the endianness of text_offset was not specified.  In
+  these cases image_size is zero and text_offset is 0x80000 in the
+  endianness of the kernel.  Where image_size is non-zero image_size is
+  little-endian and must be respected.  Where image_size is zero,
+  text_offset can be assumed to be 0x80000.
+
+- The flags field (introduced in v3.17) is a little-endian 64-bit field
+  composed as follows:
+
+  ============= ===============================================================
+  Bit 0		Kernel endianness.  1 if BE, 0 if LE.
+  Bit 1-2	Kernel Page size.
+
+			* 0 - Unspecified.
+			* 1 - 4K
+			* 2 - 16K
+			* 3 - 64K
+  Bit 3		Kernel physical placement
+
+			0
+			  2MB aligned base should be as close as possible
+			  to the base of DRAM, since memory below it is not
+			  accessible via the linear mapping
+			1
+			  2MB aligned base may be anywhere in physical
+			  memory
+  Bits 4-63	Reserved.
+  ============= ===============================================================
+
+- When image_size is zero, a bootloader should attempt to keep as much
+  memory as possible free for use by the kernel immediately after the
+  end of the kernel image. The amount of space required will vary
+  depending on selected features, and is effectively unbound.
+
+The Image must be placed text_offset bytes from a 2MB aligned base
+address anywhere in usable system RAM and called there. The region
+between the 2 MB aligned base address and the start of the image has no
+special significance to the kernel, and may be used for other purposes.
+At least image_size bytes from the start of the image must be free for
+use by the kernel.
+NOTE: versions prior to v4.6 cannot make use of memory below the
+physical offset of the Image so it is recommended that the Image be
+placed as close as possible to the start of system RAM.
+
+If an initrd/initramfs is passed to the kernel at boot, it must reside
+entirely within a 1 GB aligned physical memory window of up to 32 GB in
+size that fully covers the kernel Image as well.
+
+Any memory described to the kernel (even that below the start of the
+image) which is not marked as reserved from the kernel (e.g., with a
+memreserve region in the device tree) will be considered as available to
+the kernel.
+
+Before jumping into the kernel, the following conditions must be met:
+
+- Quiesce all DMA capable devices so that memory does not get
+  corrupted by bogus network packets or disk data.  This will save
+  you many hours of debug.
+
+- Primary CPU general-purpose register settings:
+
+    - x0 = physical address of device tree blob (dtb) in system RAM.
+    - x1 = 0 (reserved for future use)
+    - x2 = 0 (reserved for future use)
+    - x3 = 0 (reserved for future use)
+
+- CPU mode
+
+  All forms of interrupts must be masked in PSTATE.DAIF (Debug, SError,
+  IRQ and FIQ).
+  The CPU must be in either EL2 (RECOMMENDED in order to have access to
+  the virtualisation extensions) or non-secure EL1.
+
+- Caches, MMUs
+
+  The MMU must be off.
+
+  The instruction cache may be on or off, and must not hold any stale
+  entries corresponding to the loaded kernel image.
+
+  The address range corresponding to the loaded kernel image must be
+  cleaned to the PoC. In the presence of a system cache or other
+  coherent masters with caches enabled, this will typically require
+  cache maintenance by VA rather than set/way operations.
+  System caches which respect the architected cache maintenance by VA
+  operations must be configured and may be enabled.
+  System caches which do not respect architected cache maintenance by VA
+  operations (not recommended) must be configured and disabled.
+
+- Architected timers
+
+  CNTFRQ must be programmed with the timer frequency and CNTVOFF must
+  be programmed with a consistent value on all CPUs.  If entering the
+  kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0) set where
+  available.
+
+- Coherency
+
+  All CPUs to be booted by the kernel must be part of the same coherency
+  domain on entry to the kernel.  This may require IMPLEMENTATION DEFINED
+  initialisation to enable the receiving of maintenance operations on
+  each CPU.
+
+- System registers
+
+  All writable architected system registers at the exception level where
+  the kernel image will be entered must be initialised by software at a
+  higher exception level to prevent execution in an UNKNOWN state.
+
+  - SCR_EL3.FIQ must have the same value across all CPUs the kernel is
+    executing on.
+  - The value of SCR_EL3.FIQ must be the same as the one present at boot
+    time whenever the kernel is executing.
+
+  For systems with a GICv3 interrupt controller to be used in v3 mode:
+  - If EL3 is present:
+
+      - ICC_SRE_EL3.Enable (bit 3) must be initialiased to 0b1.
+      - ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b1.
+      - ICC_CTLR_EL3.PMHE (bit 6) must be set to the same value across
+        all CPUs the kernel is executing on, and must stay constant
+        for the lifetime of the kernel.
+
+  - If the kernel is entered at EL1:
+
+      - ICC.SRE_EL2.Enable (bit 3) must be initialised to 0b1
+      - ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b1.
+
+  - The DT or ACPI tables must describe a GICv3 interrupt controller.
+
+  For systems with a GICv3 interrupt controller to be used in
+  compatibility (v2) mode:
+
+  - If EL3 is present:
+
+      ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b0.
+
+  - If the kernel is entered at EL1:
+
+      ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b0.
+
+  - The DT or ACPI tables must describe a GICv2 interrupt controller.
+
+  For CPUs with pointer authentication functionality:
+
+  - If EL3 is present:
+
+    - SCR_EL3.APK (bit 16) must be initialised to 0b1
+    - SCR_EL3.API (bit 17) must be initialised to 0b1
+
+  - If the kernel is entered at EL1:
+
+    - HCR_EL2.APK (bit 40) must be initialised to 0b1
+    - HCR_EL2.API (bit 41) must be initialised to 0b1
+
+  For CPUs with Activity Monitors Unit v1 (AMUv1) extension present:
+
+  - If EL3 is present:
+
+    - CPTR_EL3.TAM (bit 30) must be initialised to 0b0
+    - CPTR_EL2.TAM (bit 30) must be initialised to 0b0
+    - AMCNTENSET0_EL0 must be initialised to 0b1111
+    - AMCNTENSET1_EL0 must be initialised to a platform specific value
+      having 0b1 set for the corresponding bit for each of the auxiliary
+      counters present.
+
+  - If the kernel is entered at EL1:
+
+    - AMCNTENSET0_EL0 must be initialised to 0b1111
+    - AMCNTENSET1_EL0 must be initialised to a platform specific value
+      having 0b1 set for the corresponding bit for each of the auxiliary
+      counters present.
+
+The requirements described above for CPU mode, caches, MMUs, architected
+timers, coherency and system registers apply to all CPUs.  All CPUs must
+enter the kernel in the same exception level.
+
+The boot loader is expected to enter the kernel on each CPU in the
+following manner:
+
+- The primary CPU must jump directly to the first instruction of the
+  kernel image.  The device tree blob passed by this CPU must contain
+  an 'enable-method' property for each cpu node.  The supported
+  enable-methods are described below.
+
+  It is expected that the bootloader will generate these device tree
+  properties and insert them into the blob prior to kernel entry.
+
+- CPUs with a "spin-table" enable-method must have a 'cpu-release-addr'
+  property in their cpu node.  This property identifies a
+  naturally-aligned 64-bit zero-initalised memory location.
+
+  These CPUs should spin outside of the kernel in a reserved area of
+  memory (communicated to the kernel by a /memreserve/ region in the
+  device tree) polling their cpu-release-addr location, which must be
+  contained in the reserved region.  A wfe instruction may be inserted
+  to reduce the overhead of the busy-loop and a sev will be issued by
+  the primary CPU.  When a read of the location pointed to by the
+  cpu-release-addr returns a non-zero value, the CPU must jump to this
+  value.  The value will be written as a single 64-bit little-endian
+  value, so CPUs must convert the read value to their native endianness
+  before jumping to it.
+
+- CPUs with a "psci" enable method should remain outside of
+  the kernel (i.e. outside of the regions of memory described to the
+  kernel in the memory node, or in a reserved area of memory described
+  to the kernel by a /memreserve/ region in the device tree).  The
+  kernel will issue CPU_ON calls as described in ARM document number ARM
+  DEN 0022A ("Power State Coordination Interface System Software on ARM
+  processors") to bring CPUs into the kernel.
+
+  The device tree should contain a 'psci' node, as described in
+  Documentation/devicetree/bindings/arm/psci.yaml.
+
+- Secondary CPU general-purpose register settings
+
+  - x0 = 0 (reserved for future use)
+  - x1 = 0 (reserved for future use)
+  - x2 = 0 (reserved for future use)
+  - x3 = 0 (reserved for future use)