1===================== 2Booting AArch64 Linux 3===================== 4 5Author: Will Deacon <will.deacon@arm.com> 6 7Date : 07 September 2012 8 9This document is based on the ARM booting document by Russell King and 10is relevant to all public releases of the AArch64 Linux kernel. 11 12The AArch64 exception model is made up of a number of exception levels 13(EL0 - EL3), with EL0 and EL1 having a secure and a non-secure 14counterpart. EL2 is the hypervisor level and exists only in non-secure 15mode. EL3 is the highest priority level and exists only in secure mode. 16 17For the purposes of this document, we will use the term `boot loader` 18simply to define all software that executes on the CPU(s) before control 19is passed to the Linux kernel. This may include secure monitor and 20hypervisor code, or it may just be a handful of instructions for 21preparing a minimal boot environment. 22 23Essentially, the boot loader should provide (as a minimum) the 24following: 25 261. Setup and initialise the RAM 272. Setup the device tree 283. Decompress the kernel image 294. Call the kernel image 30 31 321. Setup and initialise RAM 33--------------------------- 34 35Requirement: MANDATORY 36 37The boot loader is expected to find and initialise all RAM that the 38kernel will use for volatile data storage in the system. It performs 39this in a machine dependent manner. (It may use internal algorithms 40to automatically locate and size all RAM, or it may use knowledge of 41the RAM in the machine, or any other method the boot loader designer 42sees fit.) 43 44 452. Setup the device tree 46------------------------- 47 48Requirement: MANDATORY 49 50The device tree blob (dtb) must be placed on an 8-byte boundary and must 51not exceed 2 megabytes in size. Since the dtb will be mapped cacheable 52using blocks of up to 2 megabytes in size, it must not be placed within 53any 2M region which must be mapped with any specific attributes. 54 55NOTE: versions prior to v4.2 also require that the DTB be placed within 56the 512 MB region starting at text_offset bytes below the kernel Image. 57 583. Decompress the kernel image 59------------------------------ 60 61Requirement: OPTIONAL 62 63The AArch64 kernel does not currently provide a decompressor and 64therefore requires decompression (gzip etc.) to be performed by the boot 65loader if a compressed Image target (e.g. Image.gz) is used. For 66bootloaders that do not implement this requirement, the uncompressed 67Image target is available instead. 68 69 704. Call the kernel image 71------------------------ 72 73Requirement: MANDATORY 74 75The decompressed kernel image contains a 64-byte header as follows:: 76 77 u32 code0; /* Executable code */ 78 u32 code1; /* Executable code */ 79 u64 text_offset; /* Image load offset, little endian */ 80 u64 image_size; /* Effective Image size, little endian */ 81 u64 flags; /* kernel flags, little endian */ 82 u64 res2 = 0; /* reserved */ 83 u64 res3 = 0; /* reserved */ 84 u64 res4 = 0; /* reserved */ 85 u32 magic = 0x644d5241; /* Magic number, little endian, "ARM\x64" */ 86 u32 res5; /* reserved (used for PE COFF offset) */ 87 88 89Header notes: 90 91- As of v3.17, all fields are little endian unless stated otherwise. 92 93- code0/code1 are responsible for branching to stext. 94 95- when booting through EFI, code0/code1 are initially skipped. 96 res5 is an offset to the PE header and the PE header has the EFI 97 entry point (efi_stub_entry). When the stub has done its work, it 98 jumps to code0 to resume the normal boot process. 99 100- Prior to v3.17, the endianness of text_offset was not specified. In 101 these cases image_size is zero and text_offset is 0x80000 in the 102 endianness of the kernel. Where image_size is non-zero image_size is 103 little-endian and must be respected. Where image_size is zero, 104 text_offset can be assumed to be 0x80000. 105 106- The flags field (introduced in v3.17) is a little-endian 64-bit field 107 composed as follows: 108 109 ============= =============================================================== 110 Bit 0 Kernel endianness. 1 if BE, 0 if LE. 111 Bit 1-2 Kernel Page size. 112 113 * 0 - Unspecified. 114 * 1 - 4K 115 * 2 - 16K 116 * 3 - 64K 117 Bit 3 Kernel physical placement 118 119 0 120 2MB aligned base should be as close as possible 121 to the base of DRAM, since memory below it is not 122 accessible via the linear mapping 123 1 124 2MB aligned base may be anywhere in physical 125 memory 126 Bits 4-63 Reserved. 127 ============= =============================================================== 128 129- When image_size is zero, a bootloader should attempt to keep as much 130 memory as possible free for use by the kernel immediately after the 131 end of the kernel image. The amount of space required will vary 132 depending on selected features, and is effectively unbound. 133 134The Image must be placed text_offset bytes from a 2MB aligned base 135address anywhere in usable system RAM and called there. The region 136between the 2 MB aligned base address and the start of the image has no 137special significance to the kernel, and may be used for other purposes. 138At least image_size bytes from the start of the image must be free for 139use by the kernel. 140NOTE: versions prior to v4.6 cannot make use of memory below the 141physical offset of the Image so it is recommended that the Image be 142placed as close as possible to the start of system RAM. 143 144If an initrd/initramfs is passed to the kernel at boot, it must reside 145entirely within a 1 GB aligned physical memory window of up to 32 GB in 146size that fully covers the kernel Image as well. 147 148Any memory described to the kernel (even that below the start of the 149image) which is not marked as reserved from the kernel (e.g., with a 150memreserve region in the device tree) will be considered as available to 151the kernel. 152 153Before jumping into the kernel, the following conditions must be met: 154 155- Quiesce all DMA capable devices so that memory does not get 156 corrupted by bogus network packets or disk data. This will save 157 you many hours of debug. 158 159- Primary CPU general-purpose register settings: 160 161 - x0 = physical address of device tree blob (dtb) in system RAM. 162 - x1 = 0 (reserved for future use) 163 - x2 = 0 (reserved for future use) 164 - x3 = 0 (reserved for future use) 165 166- CPU mode 167 168 All forms of interrupts must be masked in PSTATE.DAIF (Debug, SError, 169 IRQ and FIQ). 170 The CPU must be in either EL2 (RECOMMENDED in order to have access to 171 the virtualisation extensions) or non-secure EL1. 172 173- Caches, MMUs 174 175 The MMU must be off. 176 Instruction cache may be on or off. 177 The address range corresponding to the loaded kernel image must be 178 cleaned to the PoC. In the presence of a system cache or other 179 coherent masters with caches enabled, this will typically require 180 cache maintenance by VA rather than set/way operations. 181 System caches which respect the architected cache maintenance by VA 182 operations must be configured and may be enabled. 183 System caches which do not respect architected cache maintenance by VA 184 operations (not recommended) must be configured and disabled. 185 186- Architected timers 187 188 CNTFRQ must be programmed with the timer frequency and CNTVOFF must 189 be programmed with a consistent value on all CPUs. If entering the 190 kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0) set where 191 available. 192 193- Coherency 194 195 All CPUs to be booted by the kernel must be part of the same coherency 196 domain on entry to the kernel. This may require IMPLEMENTATION DEFINED 197 initialisation to enable the receiving of maintenance operations on 198 each CPU. 199 200- System registers 201 202 All writable architected system registers at the exception level where 203 the kernel image will be entered must be initialised by software at a 204 higher exception level to prevent execution in an UNKNOWN state. 205 206 - SCR_EL3.FIQ must have the same value across all CPUs the kernel is 207 executing on. 208 - The value of SCR_EL3.FIQ must be the same as the one present at boot 209 time whenever the kernel is executing. 210 211 For systems with a GICv3 interrupt controller to be used in v3 mode: 212 - If EL3 is present: 213 214 - ICC_SRE_EL3.Enable (bit 3) must be initialiased to 0b1. 215 - ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b1. 216 217 - If the kernel is entered at EL1: 218 219 - ICC.SRE_EL2.Enable (bit 3) must be initialised to 0b1 220 - ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b1. 221 222 - The DT or ACPI tables must describe a GICv3 interrupt controller. 223 224 For systems with a GICv3 interrupt controller to be used in 225 compatibility (v2) mode: 226 227 - If EL3 is present: 228 229 ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b0. 230 231 - If the kernel is entered at EL1: 232 233 ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b0. 234 235 - The DT or ACPI tables must describe a GICv2 interrupt controller. 236 237 For CPUs with pointer authentication functionality: 238 - If EL3 is present: 239 240 - SCR_EL3.APK (bit 16) must be initialised to 0b1 241 - SCR_EL3.API (bit 17) must be initialised to 0b1 242 243 - If the kernel is entered at EL1: 244 245 - HCR_EL2.APK (bit 40) must be initialised to 0b1 246 - HCR_EL2.API (bit 41) must be initialised to 0b1 247 248The requirements described above for CPU mode, caches, MMUs, architected 249timers, coherency and system registers apply to all CPUs. All CPUs must 250enter the kernel in the same exception level. 251 252The boot loader is expected to enter the kernel on each CPU in the 253following manner: 254 255- The primary CPU must jump directly to the first instruction of the 256 kernel image. The device tree blob passed by this CPU must contain 257 an 'enable-method' property for each cpu node. The supported 258 enable-methods are described below. 259 260 It is expected that the bootloader will generate these device tree 261 properties and insert them into the blob prior to kernel entry. 262 263- CPUs with a "spin-table" enable-method must have a 'cpu-release-addr' 264 property in their cpu node. This property identifies a 265 naturally-aligned 64-bit zero-initalised memory location. 266 267 These CPUs should spin outside of the kernel in a reserved area of 268 memory (communicated to the kernel by a /memreserve/ region in the 269 device tree) polling their cpu-release-addr location, which must be 270 contained in the reserved region. A wfe instruction may be inserted 271 to reduce the overhead of the busy-loop and a sev will be issued by 272 the primary CPU. When a read of the location pointed to by the 273 cpu-release-addr returns a non-zero value, the CPU must jump to this 274 value. The value will be written as a single 64-bit little-endian 275 value, so CPUs must convert the read value to their native endianness 276 before jumping to it. 277 278- CPUs with a "psci" enable method should remain outside of 279 the kernel (i.e. outside of the regions of memory described to the 280 kernel in the memory node, or in a reserved area of memory described 281 to the kernel by a /memreserve/ region in the device tree). The 282 kernel will issue CPU_ON calls as described in ARM document number ARM 283 DEN 0022A ("Power State Coordination Interface System Software on ARM 284 processors") to bring CPUs into the kernel. 285 286 The device tree should contain a 'psci' node, as described in 287 Documentation/devicetree/bindings/arm/psci.yaml. 288 289- Secondary CPU general-purpose register settings 290 x0 = 0 (reserved for future use) 291 x1 = 0 (reserved for future use) 292 x2 = 0 (reserved for future use) 293 x3 = 0 (reserved for future use) 294
