Documentation/arm64/cpu-feature-registers.rst

   1 ===========================
   2 ARM64 CPU Feature Registers
   3 ===========================
   4
   5 Author: Suzuki K Poulose <suzuki.poulose@arm.com>
   6
   7
   8 This file describes the ABI for exporting the AArch64 CPU ID/feature
   9 registers to userspace. The availability of this ABI is advertised
  10 via the HWCAP_CPUID in HWCAPs.
  11
  12 1. Motivation
  13 -------------
  14
  15 The ARM architecture defines a set of feature registers, which describe
  16 the capabilities of the CPU/system. Access to these system registers is
  17 restricted from EL0 and there is no reliable way for an application to
  18 extract this information to make better decisions at runtime. There is
  19 limited information available to the application via HWCAPs, however
  20 there are some issues with their usage.
  21
  22  a) Any change to the HWCAPs requires an update to userspace (e.g libc)
  23     to detect the new changes, which can take a long time to appear in
  24     distributions. Exposing the registers allows applications to get the
  25     information without requiring updates to the toolchains.
  26
  27  b) Access to HWCAPs is sometimes limited (e.g prior to libc, or
  28     when ld is initialised at startup time).
  29
  30  c) HWCAPs cannot represent non-boolean information effectively. The
  31     architecture defines a canonical format for representing features
  32     in the ID registers; this is well defined and is capable of
  33     representing all valid architecture variations.
  34
  35
  36 2. Requirements
  37 ---------------
  38
  39  a) Safety:
  40
  41     Applications should be able to use the information provided by the
  42     infrastructure to run safely across the system. This has greater
  43     implications on a system with heterogeneous CPUs.
  44     The infrastructure exports a value that is safe across all the
  45     available CPU on the system.
  46
  47     e.g, If at least one CPU doesn't implement CRC32 instructions, while
  48     others do, we should report that the CRC32 is not implemented.
  49     Otherwise an application could crash when scheduled on the CPU
  50     which doesn't support CRC32.
  51
  52  b) Security:
  53
  54     Applications should only be able to receive information that is
  55     relevant to the normal operation in userspace. Hence, some of the
  56     fields are masked out(i.e, made invisible) and their values are set to
  57     indicate the feature is 'not supported'. See Section 4 for the list
  58     of visible features. Also, the kernel may manipulate the fields
  59     based on what it supports. e.g, If FP is not supported by the
  60     kernel, the values could indicate that the FP is not available
  61     (even when the CPU provides it).
  62
  63  c) Implementation Defined Features
  64
  65     The infrastructure doesn't expose any register which is
  66     IMPLEMENTATION DEFINED as per ARMv8-A Architecture.
  67
  68  d) CPU Identification:
  69
  70     MIDR_EL1 is exposed to help identify the processor. On a
  71     heterogeneous system, this could be racy (just like getcpu()). The
  72     process could be migrated to another CPU by the time it uses the
  73     register value, unless the CPU affinity is set. Hence, there is no
  74     guarantee that the value reflects the processor that it is
  75     currently executing on. The REVIDR is not exposed due to this
  76     constraint, as REVIDR makes sense only in conjunction with the
  77     MIDR. Alternately, MIDR_EL1 and REVIDR_EL1 are exposed via sysfs
  78     at::
  79
  80         /sys/devices/system/cpu/cpu$ID/regs/identification/
  81                                                       \- midr
  82                                                       \- revidr
  83
  84 3. Implementation
  85 --------------------
  86
  87 The infrastructure is built on the emulation of the 'MRS' instruction.
  88 Accessing a restricted system register from an application generates an
  89 exception and ends up in SIGILL being delivered to the process.
  90 The infrastructure hooks into the exception handler and emulates the
  91 operation if the source belongs to the supported system register space.
  92
  93 The infrastructure emulates only the following system register space::
  94
  95         Op0=3, Op1=0, CRn=0, CRm=0,4,5,6,7
  96
  97 (See Table C5-6 'System instruction encodings for non-Debug System
  98 register accesses' in ARMv8 ARM DDI 0487A.h, for the list of
  99 registers).
 100
 101 The following rules are applied to the value returned by the
 102 infrastructure:
 103
 104  a) The value of an 'IMPLEMENTATION DEFINED' field is set to 0.
 105  b) The value of a reserved field is populated with the reserved
 106     value as defined by the architecture.
 107  c) The value of a 'visible' field holds the system wide safe value
 108     for the particular feature (except for MIDR_EL1, see section 4).
 109  d) All other fields (i.e, invisible fields) are set to indicate
 110     the feature is missing (as defined by the architecture).
 111
 112 4. List of registers with visible features
 113 -------------------------------------------
 114
 115   1) ID_AA64ISAR0_EL1 - Instruction Set Attribute Register 0
 116
 117      +------------------------------+---------+---------+
 118      | Name                         |  bits   | visible |
 119      +------------------------------+---------+---------+
 120      | RNDR                         | [63-60] |    y    |
 121      +------------------------------+---------+---------+
 122      | TS                           | [55-52] |    y    |
 123      +------------------------------+---------+---------+
 124      | FHM                          | [51-48] |    y    |
 125      +------------------------------+---------+---------+
 126      | DP                           | [47-44] |    y    |
 127      +------------------------------+---------+---------+
 128      | SM4                          | [43-40] |    y    |
 129      +------------------------------+---------+---------+
 130      | SM3                          | [39-36] |    y    |
 131      +------------------------------+---------+---------+
 132      | SHA3                         | [35-32] |    y    |
 133      +------------------------------+---------+---------+
 134      | RDM                          | [31-28] |    y    |
 135      +------------------------------+---------+---------+
 136      | ATOMICS                      | [23-20] |    y    |
 137      +------------------------------+---------+---------+
 138      | CRC32                        | [19-16] |    y    |
 139      +------------------------------+---------+---------+
 140      | SHA2                         | [15-12] |    y    |
 141      +------------------------------+---------+---------+
 142      | SHA1                         | [11-8]  |    y    |
 143      +------------------------------+---------+---------+
 144      | AES                          | [7-4]   |    y    |
 145      +------------------------------+---------+---------+
 146
 147
 148   2) ID_AA64PFR0_EL1 - Processor Feature Register 0
 149
 150      +------------------------------+---------+---------+
 151      | Name                         |  bits   | visible |
 152      +------------------------------+---------+---------+
 153      | DIT                          | [51-48] |    y    |
 154      +------------------------------+---------+---------+
 155      | SVE                          | [35-32] |    y    |
 156      +------------------------------+---------+---------+
 157      | GIC                          | [27-24] |    n    |
 158      +------------------------------+---------+---------+
 159      | AdvSIMD                      | [23-20] |    y    |
 160      +------------------------------+---------+---------+
 161      | FP                           | [19-16] |    y    |
 162      +------------------------------+---------+---------+
 163      | EL3                          | [15-12] |    n    |
 164      +------------------------------+---------+---------+
 165      | EL2                          | [11-8]  |    n    |
 166      +------------------------------+---------+---------+
 167      | EL1                          | [7-4]   |    n    |
 168      +------------------------------+---------+---------+
 169      | EL0                          | [3-0]   |    n    |
 170      +------------------------------+---------+---------+
 171
 172
 173   3) ID_AA64PFR1_EL1 - Processor Feature Register 1
 174
 175      +------------------------------+---------+---------+
 176      | Name                         |  bits   | visible |
 177      +------------------------------+---------+---------+
 178      | MTE                          | [11-8]  |    y    |
 179      +------------------------------+---------+---------+
 180      | SSBS                         | [7-4]   |    y    |
 181      +------------------------------+---------+---------+
 182      | BT                           | [3-0]   |    y    |
 183      +------------------------------+---------+---------+
 184
 185
 186   4) MIDR_EL1 - Main ID Register
 187
 188      +------------------------------+---------+---------+
 189      | Name                         |  bits   | visible |
 190      +------------------------------+---------+---------+
 191      | Implementer                  | [31-24] |    y    |
 192      +------------------------------+---------+---------+
 193      | Variant                      | [23-20] |    y    |
 194      +------------------------------+---------+---------+
 195      | Architecture                 | [19-16] |    y    |
 196      +------------------------------+---------+---------+
 197      | PartNum                      | [15-4]  |    y    |
 198      +------------------------------+---------+---------+
 199      | Revision                     | [3-0]   |    y    |
 200      +------------------------------+---------+---------+
 201
 202    NOTE: The 'visible' fields of MIDR_EL1 will contain the value
 203    as available on the CPU where it is fetched and is not a system
 204    wide safe value.
 205
 206   5) ID_AA64ISAR1_EL1 - Instruction set attribute register 1
 207
 208      +------------------------------+---------+---------+
 209      | Name                         |  bits   | visible |
 210      +------------------------------+---------+---------+
 211      | I8MM                         | [55-52] |    y    |
 212      +------------------------------+---------+---------+
 213      | DGH                          | [51-48] |    y    |
 214      +------------------------------+---------+---------+
 215      | BF16                         | [47-44] |    y    |
 216      +------------------------------+---------+---------+
 217      | SB                           | [39-36] |    y    |
 218      +------------------------------+---------+---------+
 219      | FRINTTS                      | [35-32] |    y    |
 220      +------------------------------+---------+---------+
 221      | GPI                          | [31-28] |    y    |
 222      +------------------------------+---------+---------+
 223      | GPA                          | [27-24] |    y    |
 224      +------------------------------+---------+---------+
 225      | LRCPC                        | [23-20] |    y    |
 226      +------------------------------+---------+---------+
 227      | FCMA                         | [19-16] |    y    |
 228      +------------------------------+---------+---------+
 229      | JSCVT                        | [15-12] |    y    |
 230      +------------------------------+---------+---------+
 231      | API                          | [11-8]  |    y    |
 232      +------------------------------+---------+---------+
 233      | APA                          | [7-4]   |    y    |
 234      +------------------------------+---------+---------+
 235      | DPB                          | [3-0]   |    y    |
 236      +------------------------------+---------+---------+
 237
 238   6) ID_AA64MMFR2_EL1 - Memory model feature register 2
 239
 240      +------------------------------+---------+---------+
 241      | Name                         |  bits   | visible |
 242      +------------------------------+---------+---------+
 243      | AT                           | [35-32] |    y    |
 244      +------------------------------+---------+---------+
 245
 246   7) ID_AA64ZFR0_EL1 - SVE feature ID register 0
 247
 248      +------------------------------+---------+---------+
 249      | Name                         |  bits   | visible |
 250      +------------------------------+---------+---------+
 251      | F64MM                        | [59-56] |    y    |
 252      +------------------------------+---------+---------+
 253      | F32MM                        | [55-52] |    y    |
 254      +------------------------------+---------+---------+
 255      | I8MM                         | [47-44] |    y    |
 256      +------------------------------+---------+---------+
 257      | SM4                          | [43-40] |    y    |
 258      +------------------------------+---------+---------+
 259      | SHA3                         | [35-32] |    y    |
 260      +------------------------------+---------+---------+
 261      | BF16                         | [23-20] |    y    |
 262      +------------------------------+---------+---------+
 263      | BitPerm                      | [19-16] |    y    |
 264      +------------------------------+---------+---------+
 265      | AES                          | [7-4]   |    y    |
 266      +------------------------------+---------+---------+
 267      | SVEVer                       | [3-0]   |    y    |
 268      +------------------------------+---------+---------+
 269
 270 Appendix I: Example
 271 -------------------
 272
 273 ::
 274
 275   /*
 276    * Sample program to demonstrate the MRS emulation ABI.
 277    *
 278    * Copyright (C) 2015-2016, ARM Ltd
 279    *
 280    * Author: Suzuki K Poulose <suzuki.poulose@arm.com>
 281    *
 282    * This program is free software; you can redistribute it and/or modify
 283    * it under the terms of the GNU General Public License version 2 as
 284    * published by the Free Software Foundation.
 285    *
 286    * This program is distributed in the hope that it will be useful,
 287    * but WITHOUT ANY WARRANTY; without even the implied warranty of
 288    * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 289    * GNU General Public License for more details.
 290    * This program is free software; you can redistribute it and/or modify
 291    * it under the terms of the GNU General Public License version 2 as
 292    * published by the Free Software Foundation.
 293    *
 294    * This program is distributed in the hope that it will be useful,
 295    * but WITHOUT ANY WARRANTY; without even the implied warranty of
 296    * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 297    * GNU General Public License for more details.
 298    */
 299
 300   #include <asm/hwcap.h>
 301   #include <stdio.h>
 302   #include <sys/auxv.h>
 303
 304   #define get_cpu_ftr(id) ({                                    \
 305                 unsigned long __val;                            \
 306                 asm("mrs %0, "#id : "=r" (__val));              \
 307                 printf("%-20s: 0x%016lx\n", #id, __val);        \
 308         })
 309
 310   int main(void)
 311   {
 312
 313         if (!(getauxval(AT_HWCAP) & HWCAP_CPUID)) {
 314                 fputs("CPUID registers unavailable\n", stderr);
 315                 return 1;
 316         }
 317
 318         get_cpu_ftr(ID_AA64ISAR0_EL1);
 319         get_cpu_ftr(ID_AA64ISAR1_EL1);
 320         get_cpu_ftr(ID_AA64MMFR0_EL1);
 321         get_cpu_ftr(ID_AA64MMFR1_EL1);
 322         get_cpu_ftr(ID_AA64PFR0_EL1);
 323         get_cpu_ftr(ID_AA64PFR1_EL1);
 324         get_cpu_ftr(ID_AA64DFR0_EL1);
 325         get_cpu_ftr(ID_AA64DFR1_EL1);
 326
 327         get_cpu_ftr(MIDR_EL1);
 328         get_cpu_ftr(MPIDR_EL1);
 329         get_cpu_ftr(REVIDR_EL1);
 330
 331   #if 0
 332         /* Unexposed register access causes SIGILL */
 333         get_cpu_ftr(ID_MMFR0_EL1);
 334   #endif
 335
 336         return 0;
 337   }