Documentation/arm64/cpu-feature-registers.txt

   1                 ARM64 CPU Feature Registers
   2                 ===========================
   3
   4 Author: Suzuki K Poulose <suzuki.poulose@arm.com>
   5
   6
   7 This file describes the ABI for exporting the AArch64 CPU ID/feature
   8 registers to userspace. The availability of this ABI is advertised
   9 via the HWCAP_CPUID in HWCAPs.
  10
  11 1. Motivation
  12 ---------------
  13
  14 The ARM architecture defines a set of feature registers, which describe
  15 the capabilities of the CPU/system. Access to these system registers is
  16 restricted from EL0 and there is no reliable way for an application to
  17 extract this information to make better decisions at runtime. There is
  18 limited information available to the application via HWCAPs, however
  19 there are some issues with their usage.
  20
  21  a) Any change to the HWCAPs requires an update to userspace (e.g libc)
  22     to detect the new changes, which can take a long time to appear in
  23     distributions. Exposing the registers allows applications to get the
  24     information without requiring updates to the toolchains.
  25
  26  b) Access to HWCAPs is sometimes limited (e.g prior to libc, or
  27     when ld is initialised at startup time).
  28
  29  c) HWCAPs cannot represent non-boolean information effectively. The
  30     architecture defines a canonical format for representing features
  31     in the ID registers; this is well defined and is capable of
  32     representing all valid architecture variations.
  33
  34
  35 2. Requirements
  36 -----------------
  37
  38  a) Safety :
  39     Applications should be able to use the information provided by the
  40     infrastructure to run safely across the system. This has greater
  41     implications on a system with heterogeneous CPUs.
  42     The infrastructure exports a value that is safe across all the
  43     available CPU on the system.
  44
  45     e.g, If at least one CPU doesn't implement CRC32 instructions, while
  46     others do, we should report that the CRC32 is not implemented.
  47     Otherwise an application could crash when scheduled on the CPU
  48     which doesn't support CRC32.
  49
  50  b) Security :
  51     Applications should only be able to receive information that is
  52     relevant to the normal operation in userspace. Hence, some of the
  53     fields are masked out(i.e, made invisible) and their values are set to
  54     indicate the feature is 'not supported'. See Section 4 for the list
  55     of visible features. Also, the kernel may manipulate the fields
  56     based on what it supports. e.g, If FP is not supported by the
  57     kernel, the values could indicate that the FP is not available
  58     (even when the CPU provides it).
  59
  60  c) Implementation Defined Features
  61     The infrastructure doesn't expose any register which is
  62     IMPLEMENTATION DEFINED as per ARMv8-A Architecture.
  63
  64  d) CPU Identification :
  65     MIDR_EL1 is exposed to help identify the processor. On a
  66     heterogeneous system, this could be racy (just like getcpu()). The
  67     process could be migrated to another CPU by the time it uses the
  68     register value, unless the CPU affinity is set. Hence, there is no
  69     guarantee that the value reflects the processor that it is
  70     currently executing on. The REVIDR is not exposed due to this
  71     constraint, as REVIDR makes sense only in conjunction with the
  72     MIDR. Alternately, MIDR_EL1 and REVIDR_EL1 are exposed via sysfs
  73     at:
  74
  75         /sys/devices/system/cpu/cpu$ID/regs/identification/
  76                                                       \- midr
  77                                                       \- revidr
  78
  79 3. Implementation
  80 --------------------
  81
  82 The infrastructure is built on the emulation of the 'MRS' instruction.
  83 Accessing a restricted system register from an application generates an
  84 exception and ends up in SIGILL being delivered to the process.
  85 The infrastructure hooks into the exception handler and emulates the
  86 operation if the source belongs to the supported system register space.
  87
  88 The infrastructure emulates only the following system register space:
  89         Op0=3, Op1=0, CRn=0, CRm=0,4,5,6,7
  90
  91 (See Table C5-6 'System instruction encodings for non-Debug System
  92 register accesses' in ARMv8 ARM DDI 0487A.h, for the list of
  93 registers).
  94
  95 The following rules are applied to the value returned by the
  96 infrastructure:
  97
  98  a) The value of an 'IMPLEMENTATION DEFINED' field is set to 0.
  99  b) The value of a reserved field is populated with the reserved
 100     value as defined by the architecture.
 101  c) The value of a 'visible' field holds the system wide safe value
 102     for the particular feature (except for MIDR_EL1, see section 4).
 103  d) All other fields (i.e, invisible fields) are set to indicate
 104     the feature is missing (as defined by the architecture).
 105
 106 4. List of registers with visible features
 107 -------------------------------------------
 108
 109   1) ID_AA64ISAR0_EL1 - Instruction Set Attribute Register 0
 110      x--------------------------------------------------x
 111      | Name                         |  bits   | visible |
 112      |--------------------------------------------------|
 113      | TS                           | [55-52] |    y    |
 114      |--------------------------------------------------|
 115      | FHM                          | [51-48] |    y    |
 116      |--------------------------------------------------|
 117      | DP                           | [47-44] |    y    |
 118      |--------------------------------------------------|
 119      | SM4                          | [43-40] |    y    |
 120      |--------------------------------------------------|
 121      | SM3                          | [39-36] |    y    |
 122      |--------------------------------------------------|
 123      | SHA3                         | [35-32] |    y    |
 124      |--------------------------------------------------|
 125      | RDM                          | [31-28] |    y    |
 126      |--------------------------------------------------|
 127      | ATOMICS                      | [23-20] |    y    |
 128      |--------------------------------------------------|
 129      | CRC32                        | [19-16] |    y    |
 130      |--------------------------------------------------|
 131      | SHA2                         | [15-12] |    y    |
 132      |--------------------------------------------------|
 133      | SHA1                         | [11-8]  |    y    |
 134      |--------------------------------------------------|
 135      | AES                          | [7-4]   |    y    |
 136      x--------------------------------------------------x
 137
 138
 139   2) ID_AA64PFR0_EL1 - Processor Feature Register 0
 140      x--------------------------------------------------x
 141      | Name                         |  bits   | visible |
 142      |--------------------------------------------------|
 143      | DIT                          | [51-48] |    y    |
 144      |--------------------------------------------------|
 145      | SVE                          | [35-32] |    y    |
 146      |--------------------------------------------------|
 147      | GIC                          | [27-24] |    n    |
 148      |--------------------------------------------------|
 149      | AdvSIMD                      | [23-20] |    y    |
 150      |--------------------------------------------------|
 151      | FP                           | [19-16] |    y    |
 152      |--------------------------------------------------|
 153      | EL3                          | [15-12] |    n    |
 154      |--------------------------------------------------|
 155      | EL2                          | [11-8]  |    n    |
 156      |--------------------------------------------------|
 157      | EL1                          | [7-4]   |    n    |
 158      |--------------------------------------------------|
 159      | EL0                          | [3-0]   |    n    |
 160      x--------------------------------------------------x
 161
 162
 163   3) MIDR_EL1 - Main ID Register
 164      x--------------------------------------------------x
 165      | Name                         |  bits   | visible |
 166      |--------------------------------------------------|
 167      | Implementer                  | [31-24] |    y    |
 168      |--------------------------------------------------|
 169      | Variant                      | [23-20] |    y    |
 170      |--------------------------------------------------|
 171      | Architecture                 | [19-16] |    y    |
 172      |--------------------------------------------------|
 173      | PartNum                      | [15-4]  |    y    |
 174      |--------------------------------------------------|
 175      | Revision                     | [3-0]   |    y    |
 176      x--------------------------------------------------x
 177
 178    NOTE: The 'visible' fields of MIDR_EL1 will contain the value
 179    as available on the CPU where it is fetched and is not a system
 180    wide safe value.
 181
 182   4) ID_AA64ISAR1_EL1 - Instruction set attribute register 1
 183
 184      x--------------------------------------------------x
 185      | Name                         |  bits   | visible |
 186      |--------------------------------------------------|
 187      | LRCPC                        | [23-20] |    y    |
 188      |--------------------------------------------------|
 189      | FCMA                         | [19-16] |    y    |
 190      |--------------------------------------------------|
 191      | JSCVT                        | [15-12] |    y    |
 192      |--------------------------------------------------|
 193      | DPB                          | [3-0]   |    y    |
 194      x--------------------------------------------------x
 195
 196   5) ID_AA64MMFR2_EL1 - Memory model feature register 2
 197
 198      x--------------------------------------------------x
 199      | Name                         |  bits   | visible |
 200      |--------------------------------------------------|
 201      | AT                           | [35-32] |    y    |
 202      x--------------------------------------------------x
 203
 204 Appendix I: Example
 205 ---------------------------
 206
 207 /*
 208  * Sample program to demonstrate the MRS emulation ABI.
 209  *
 210  * Copyright (C) 2015-2016, ARM Ltd
 211  *
 212  * Author: Suzuki K Poulose <suzuki.poulose@arm.com>
 213  *
 214  * This program is free software; you can redistribute it and/or modify
 215  * it under the terms of the GNU General Public License version 2 as
 216  * published by the Free Software Foundation.
 217  *
 218  * This program is distributed in the hope that it will be useful,
 219  * but WITHOUT ANY WARRANTY; without even the implied warranty of
 220  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 221  * GNU General Public License for more details.
 222  * This program is free software; you can redistribute it and/or modify
 223  * it under the terms of the GNU General Public License version 2 as
 224  * published by the Free Software Foundation.
 225  *
 226  * This program is distributed in the hope that it will be useful,
 227  * but WITHOUT ANY WARRANTY; without even the implied warranty of
 228  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 229  * GNU General Public License for more details.
 230  */
 231
 232 #include <asm/hwcap.h>
 233 #include <stdio.h>
 234 #include <sys/auxv.h>
 235
 236 #define get_cpu_ftr(id) ({                                      \
 237                 unsigned long __val;                            \
 238                 asm("mrs %0, "#id : "=r" (__val));              \
 239                 printf("%-20s: 0x%016lx\n", #id, __val);        \
 240         })
 241
 242 int main(void)
 243 {
 244
 245         if (!(getauxval(AT_HWCAP) & HWCAP_CPUID)) {
 246                 fputs("CPUID registers unavailable\n", stderr);
 247                 return 1;
 248         }
 249
 250         get_cpu_ftr(ID_AA64ISAR0_EL1);
 251         get_cpu_ftr(ID_AA64ISAR1_EL1);
 252         get_cpu_ftr(ID_AA64MMFR0_EL1);
 253         get_cpu_ftr(ID_AA64MMFR1_EL1);
 254         get_cpu_ftr(ID_AA64PFR0_EL1);
 255         get_cpu_ftr(ID_AA64PFR1_EL1);
 256         get_cpu_ftr(ID_AA64DFR0_EL1);
 257         get_cpu_ftr(ID_AA64DFR1_EL1);
 258
 259         get_cpu_ftr(MIDR_EL1);
 260         get_cpu_ftr(MPIDR_EL1);
 261         get_cpu_ftr(REVIDR_EL1);
 262
 263 #if 0
 264         /* Unexposed register access causes SIGILL */
 265         get_cpu_ftr(ID_MMFR0_EL1);
 266 #endif
 267
 268         return 0;
 269 }
 270
 271
 272