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 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 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)