153 lines
5.4 KiB
Plaintext
153 lines
5.4 KiB
Plaintext
Booting AArch64 Linux
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=====================
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Author: Will Deacon <will.deacon@arm.com>
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Date : 07 September 2012
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This document is based on the ARM booting document by Russell King and
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is relevant to all public releases of the AArch64 Linux kernel.
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The AArch64 exception model is made up of a number of exception levels
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(EL0 - EL3), with EL0 and EL1 having a secure and a non-secure
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counterpart. EL2 is the hypervisor level and exists only in non-secure
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mode. EL3 is the highest priority level and exists only in secure mode.
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For the purposes of this document, we will use the term `boot loader'
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simply to define all software that executes on the CPU(s) before control
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is passed to the Linux kernel. This may include secure monitor and
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hypervisor code, or it may just be a handful of instructions for
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preparing a minimal boot environment.
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Essentially, the boot loader should provide (as a minimum) the
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following:
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1. Setup and initialise the RAM
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2. Setup the device tree
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3. Decompress the kernel image
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4. Call the kernel image
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1. Setup and initialise RAM
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---------------------------
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Requirement: MANDATORY
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The boot loader is expected to find and initialise all RAM that the
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kernel will use for volatile data storage in the system. It performs
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this in a machine dependent manner. (It may use internal algorithms
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to automatically locate and size all RAM, or it may use knowledge of
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the RAM in the machine, or any other method the boot loader designer
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sees fit.)
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2. Setup the device tree
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-------------------------
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Requirement: MANDATORY
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The device tree blob (dtb) must be no bigger than 2 megabytes in size
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and placed at a 2-megabyte boundary within the first 512 megabytes from
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the start of the kernel image. This is to allow the kernel to map the
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blob using a single section mapping in the initial page tables.
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3. Decompress the kernel image
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------------------------------
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Requirement: OPTIONAL
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The AArch64 kernel does not currently provide a decompressor and
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therefore requires decompression (gzip etc.) to be performed by the boot
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loader if a compressed Image target (e.g. Image.gz) is used. For
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bootloaders that do not implement this requirement, the uncompressed
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Image target is available instead.
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4. Call the kernel image
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------------------------
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Requirement: MANDATORY
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The decompressed kernel image contains a 32-byte header as follows:
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u32 magic = 0x14000008; /* branch to stext, little-endian */
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u32 res0 = 0; /* reserved */
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u64 text_offset; /* Image load offset */
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u64 res1 = 0; /* reserved */
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u64 res2 = 0; /* reserved */
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The image must be placed at the specified offset (currently 0x80000)
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from the start of the system RAM and called there. The start of the
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system RAM must be aligned to 2MB.
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Before jumping into the kernel, the following conditions must be met:
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- Quiesce all DMA capable devices so that memory does not get
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corrupted by bogus network packets or disk data. This will save
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you many hours of debug.
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- Primary CPU general-purpose register settings
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x0 = physical address of device tree blob (dtb) in system RAM.
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x1 = 0 (reserved for future use)
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x2 = 0 (reserved for future use)
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x3 = 0 (reserved for future use)
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- CPU mode
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All forms of interrupts must be masked in PSTATE.DAIF (Debug, SError,
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IRQ and FIQ).
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The CPU must be in either EL2 (RECOMMENDED in order to have access to
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the virtualisation extensions) or non-secure EL1.
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- Caches, MMUs
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The MMU must be off.
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Instruction cache may be on or off.
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Data cache must be off and invalidated.
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External caches (if present) must be configured and disabled.
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- Architected timers
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CNTFRQ must be programmed with the timer frequency.
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If entering the kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0)
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set where available.
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- Coherency
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All CPUs to be booted by the kernel must be part of the same coherency
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domain on entry to the kernel. This may require IMPLEMENTATION DEFINED
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initialisation to enable the receiving of maintenance operations on
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each CPU.
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- System registers
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All writable architected system registers at the exception level where
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the kernel image will be entered must be initialised by software at a
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higher exception level to prevent execution in an UNKNOWN state.
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The boot loader is expected to enter the kernel on each CPU in the
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following manner:
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- The primary CPU must jump directly to the first instruction of the
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kernel image. The device tree blob passed by this CPU must contain
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for each CPU node:
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1. An 'enable-method' property. Currently, the only supported value
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for this field is the string "spin-table".
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2. A 'cpu-release-addr' property identifying a 64-bit,
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zero-initialised memory location.
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It is expected that the bootloader will generate these device tree
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properties and insert them into the blob prior to kernel entry.
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- Any secondary CPUs must spin outside of the kernel in a reserved area
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of memory (communicated to the kernel by a /memreserve/ region in the
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device tree) polling their cpu-release-addr location, which must be
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contained in the reserved region. A wfe instruction may be inserted
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to reduce the overhead of the busy-loop and a sev will be issued by
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the primary CPU. When a read of the location pointed to by the
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cpu-release-addr returns a non-zero value, the CPU must jump directly
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to this value.
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- Secondary CPU general-purpose register settings
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x0 = 0 (reserved for future use)
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x1 = 0 (reserved for future use)
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x2 = 0 (reserved for future use)
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x3 = 0 (reserved for future use)
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