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Booting AArch64 Linux
Author: Will Deacon <>
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
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 no bigger than 2 megabytes in size
and placed at a 2-megabyte boundary within the first 512 megabytes from
the start of the kernel image. 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 32-byte header as follows:
u32 magic = 0x14000008; /* branch to stext, little-endian */
u32 res0 = 0; /* reserved */
u64 text_offset; /* Image load offset */
u64 res1 = 0; /* reserved */
u64 res2 = 0; /* reserved */
The image must be placed at the specified offset (currently 0x80000)
from the start of the system RAM and called there. The start of the
system RAM must be aligned to 2MB.
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.
Data cache must be off and invalidated.
External caches (if present) must be configured and disabled.
- Architected timers
CNTFRQ must be programmed with the timer frequency.
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.
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
for each CPU node:
1. An 'enable-method' property. Currently, the only supported value
for this field is the string "spin-table".
2. A 'cpu-release-addr' property identifying a 64-bit,
zero-initialised memory location.
It is expected that the bootloader will generate these device tree
properties and insert them into the blob prior to kernel entry.
- Any secondary CPUs must 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 directly
to this value.
- 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)