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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 within
the first 512 megabytes from the start of the kernel image and must not
cross a 2-megabyte boundary. This is to allow the kernel to map the
blob using a single section mapping in the initial page tables.
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.
Bits 1-63: 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 near the start of usable system RAM and called there. Memory
below that base address is currently unusable by Linux, and therefore it
is strongly recommended that this location is the start of system RAM.
At least image_size bytes from the start of the image must be free for
use by the kernel.
Any memory described to the kernel (even that below the 2MB aligned base
address) 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.
Instruction cache may be on or off.
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.
For systems with a GICv3 interrupt controller:
- 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.
- 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 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.txt.
- 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)
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