| =========================== |
| ARM64 CPU Feature Registers |
| =========================== |
| |
| Author: Suzuki K Poulose <suzuki.poulose@arm.com> |
| |
| |
| This file describes the ABI for exporting the AArch64 CPU ID/feature |
| registers to userspace. The availability of this ABI is advertised |
| via the HWCAP_CPUID in HWCAPs. |
| |
| 1. Motivation |
| ------------- |
| |
| The ARM architecture defines a set of feature registers, which describe |
| the capabilities of the CPU/system. Access to these system registers is |
| restricted from EL0 and there is no reliable way for an application to |
| extract this information to make better decisions at runtime. There is |
| limited information available to the application via HWCAPs, however |
| there are some issues with their usage. |
| |
| a) Any change to the HWCAPs requires an update to userspace (e.g libc) |
| to detect the new changes, which can take a long time to appear in |
| distributions. Exposing the registers allows applications to get the |
| information without requiring updates to the toolchains. |
| |
| b) Access to HWCAPs is sometimes limited (e.g prior to libc, or |
| when ld is initialised at startup time). |
| |
| c) HWCAPs cannot represent non-boolean information effectively. The |
| architecture defines a canonical format for representing features |
| in the ID registers; this is well defined and is capable of |
| representing all valid architecture variations. |
| |
| |
| 2. Requirements |
| --------------- |
| |
| a) Safety: |
| |
| Applications should be able to use the information provided by the |
| infrastructure to run safely across the system. This has greater |
| implications on a system with heterogeneous CPUs. |
| The infrastructure exports a value that is safe across all the |
| available CPU on the system. |
| |
| e.g, If at least one CPU doesn't implement CRC32 instructions, while |
| others do, we should report that the CRC32 is not implemented. |
| Otherwise an application could crash when scheduled on the CPU |
| which doesn't support CRC32. |
| |
| b) Security: |
| |
| Applications should only be able to receive information that is |
| relevant to the normal operation in userspace. Hence, some of the |
| fields are masked out(i.e, made invisible) and their values are set to |
| indicate the feature is 'not supported'. See Section 4 for the list |
| of visible features. Also, the kernel may manipulate the fields |
| based on what it supports. e.g, If FP is not supported by the |
| kernel, the values could indicate that the FP is not available |
| (even when the CPU provides it). |
| |
| c) Implementation Defined Features |
| |
| The infrastructure doesn't expose any register which is |
| IMPLEMENTATION DEFINED as per ARMv8-A Architecture. |
| |
| d) CPU Identification: |
| |
| MIDR_EL1 is exposed to help identify the processor. On a |
| heterogeneous system, this could be racy (just like getcpu()). The |
| process could be migrated to another CPU by the time it uses the |
| register value, unless the CPU affinity is set. Hence, there is no |
| guarantee that the value reflects the processor that it is |
| currently executing on. The REVIDR is not exposed due to this |
| constraint, as REVIDR makes sense only in conjunction with the |
| MIDR. Alternately, MIDR_EL1 and REVIDR_EL1 are exposed via sysfs |
| at:: |
| |
| /sys/devices/system/cpu/cpu$ID/regs/identification/ |
| \- midr |
| \- revidr |
| |
| 3. Implementation |
| -------------------- |
| |
| The infrastructure is built on the emulation of the 'MRS' instruction. |
| Accessing a restricted system register from an application generates an |
| exception and ends up in SIGILL being delivered to the process. |
| The infrastructure hooks into the exception handler and emulates the |
| operation if the source belongs to the supported system register space. |
| |
| The infrastructure emulates only the following system register space:: |
| |
| Op0=3, Op1=0, CRn=0, CRm=0,4,5,6,7 |
| |
| (See Table C5-6 'System instruction encodings for non-Debug System |
| register accesses' in ARMv8 ARM DDI 0487A.h, for the list of |
| registers). |
| |
| The following rules are applied to the value returned by the |
| infrastructure: |
| |
| a) The value of an 'IMPLEMENTATION DEFINED' field is set to 0. |
| b) The value of a reserved field is populated with the reserved |
| value as defined by the architecture. |
| c) The value of a 'visible' field holds the system wide safe value |
| for the particular feature (except for MIDR_EL1, see section 4). |
| d) All other fields (i.e, invisible fields) are set to indicate |
| the feature is missing (as defined by the architecture). |
| |
| 4. List of registers with visible features |
| ------------------------------------------- |
| |
| 1) ID_AA64ISAR0_EL1 - Instruction Set Attribute Register 0 |
| |
| +------------------------------+---------+---------+ |
| | Name | bits | visible | |
| +------------------------------+---------+---------+ |
| | RNDR | [63-60] | y | |
| +------------------------------+---------+---------+ |
| | TS | [55-52] | y | |
| +------------------------------+---------+---------+ |
| | FHM | [51-48] | y | |
| +------------------------------+---------+---------+ |
| | DP | [47-44] | y | |
| +------------------------------+---------+---------+ |
| | SM4 | [43-40] | y | |
| +------------------------------+---------+---------+ |
| | SM3 | [39-36] | y | |
| +------------------------------+---------+---------+ |
| | SHA3 | [35-32] | y | |
| +------------------------------+---------+---------+ |
| | RDM | [31-28] | y | |
| +------------------------------+---------+---------+ |
| | ATOMICS | [23-20] | y | |
| +------------------------------+---------+---------+ |
| | CRC32 | [19-16] | y | |
| +------------------------------+---------+---------+ |
| | SHA2 | [15-12] | y | |
| +------------------------------+---------+---------+ |
| | SHA1 | [11-8] | y | |
| +------------------------------+---------+---------+ |
| | AES | [7-4] | y | |
| +------------------------------+---------+---------+ |
| |
| |
| 2) ID_AA64PFR0_EL1 - Processor Feature Register 0 |
| |
| +------------------------------+---------+---------+ |
| | Name | bits | visible | |
| +------------------------------+---------+---------+ |
| | DIT | [51-48] | y | |
| +------------------------------+---------+---------+ |
| | SVE | [35-32] | y | |
| +------------------------------+---------+---------+ |
| | GIC | [27-24] | n | |
| +------------------------------+---------+---------+ |
| | AdvSIMD | [23-20] | y | |
| +------------------------------+---------+---------+ |
| | FP | [19-16] | y | |
| +------------------------------+---------+---------+ |
| | EL3 | [15-12] | n | |
| +------------------------------+---------+---------+ |
| | EL2 | [11-8] | n | |
| +------------------------------+---------+---------+ |
| | EL1 | [7-4] | n | |
| +------------------------------+---------+---------+ |
| | EL0 | [3-0] | n | |
| +------------------------------+---------+---------+ |
| |
| |
| 3) ID_AA64PFR1_EL1 - Processor Feature Register 1 |
| |
| +------------------------------+---------+---------+ |
| | Name | bits | visible | |
| +------------------------------+---------+---------+ |
| | SSBS | [7-4] | y | |
| +------------------------------+---------+---------+ |
| | BT | [3-0] | y | |
| +------------------------------+---------+---------+ |
| |
| |
| 4) MIDR_EL1 - Main ID Register |
| |
| +------------------------------+---------+---------+ |
| | Name | bits | visible | |
| +------------------------------+---------+---------+ |
| | Implementer | [31-24] | y | |
| +------------------------------+---------+---------+ |
| | Variant | [23-20] | y | |
| +------------------------------+---------+---------+ |
| | Architecture | [19-16] | y | |
| +------------------------------+---------+---------+ |
| | PartNum | [15-4] | y | |
| +------------------------------+---------+---------+ |
| | Revision | [3-0] | y | |
| +------------------------------+---------+---------+ |
| |
| NOTE: The 'visible' fields of MIDR_EL1 will contain the value |
| as available on the CPU where it is fetched and is not a system |
| wide safe value. |
| |
| 5) ID_AA64ISAR1_EL1 - Instruction set attribute register 1 |
| |
| +------------------------------+---------+---------+ |
| | Name | bits | visible | |
| +------------------------------+---------+---------+ |
| | I8MM | [55-52] | y | |
| +------------------------------+---------+---------+ |
| | DGH | [51-48] | y | |
| +------------------------------+---------+---------+ |
| | BF16 | [47-44] | y | |
| +------------------------------+---------+---------+ |
| | SB | [39-36] | y | |
| +------------------------------+---------+---------+ |
| | FRINTTS | [35-32] | y | |
| +------------------------------+---------+---------+ |
| | GPI | [31-28] | y | |
| +------------------------------+---------+---------+ |
| | GPA | [27-24] | y | |
| +------------------------------+---------+---------+ |
| | LRCPC | [23-20] | y | |
| +------------------------------+---------+---------+ |
| | FCMA | [19-16] | y | |
| +------------------------------+---------+---------+ |
| | JSCVT | [15-12] | y | |
| +------------------------------+---------+---------+ |
| | API | [11-8] | y | |
| +------------------------------+---------+---------+ |
| | APA | [7-4] | y | |
| +------------------------------+---------+---------+ |
| | DPB | [3-0] | y | |
| +------------------------------+---------+---------+ |
| |
| 6) ID_AA64MMFR2_EL1 - Memory model feature register 2 |
| |
| +------------------------------+---------+---------+ |
| | Name | bits | visible | |
| +------------------------------+---------+---------+ |
| | AT | [35-32] | y | |
| +------------------------------+---------+---------+ |
| |
| 7) ID_AA64ZFR0_EL1 - SVE feature ID register 0 |
| |
| +------------------------------+---------+---------+ |
| | Name | bits | visible | |
| +------------------------------+---------+---------+ |
| | F64MM | [59-56] | y | |
| +------------------------------+---------+---------+ |
| | F32MM | [55-52] | y | |
| +------------------------------+---------+---------+ |
| | I8MM | [47-44] | y | |
| +------------------------------+---------+---------+ |
| | SM4 | [43-40] | y | |
| +------------------------------+---------+---------+ |
| | SHA3 | [35-32] | y | |
| +------------------------------+---------+---------+ |
| | BF16 | [23-20] | y | |
| +------------------------------+---------+---------+ |
| | BitPerm | [19-16] | y | |
| +------------------------------+---------+---------+ |
| | AES | [7-4] | y | |
| +------------------------------+---------+---------+ |
| | SVEVer | [3-0] | y | |
| +------------------------------+---------+---------+ |
| |
| Appendix I: Example |
| ------------------- |
| |
| :: |
| |
| /* |
| * Sample program to demonstrate the MRS emulation ABI. |
| * |
| * Copyright (C) 2015-2016, ARM Ltd |
| * |
| * Author: Suzuki K Poulose <suzuki.poulose@arm.com> |
| * |
| * This program is free software; you can redistribute it and/or modify |
| * it under the terms of the GNU General Public License version 2 as |
| * published by the Free Software Foundation. |
| * |
| * This program is distributed in the hope that it will be useful, |
| * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| * GNU General Public License for more details. |
| * This program is free software; you can redistribute it and/or modify |
| * it under the terms of the GNU General Public License version 2 as |
| * published by the Free Software Foundation. |
| * |
| * This program is distributed in the hope that it will be useful, |
| * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| * GNU General Public License for more details. |
| */ |
| |
| #include <asm/hwcap.h> |
| #include <stdio.h> |
| #include <sys/auxv.h> |
| |
| #define get_cpu_ftr(id) ({ \ |
| unsigned long __val; \ |
| asm("mrs %0, "#id : "=r" (__val)); \ |
| printf("%-20s: 0x%016lx\n", #id, __val); \ |
| }) |
| |
| int main(void) |
| { |
| |
| if (!(getauxval(AT_HWCAP) & HWCAP_CPUID)) { |
| fputs("CPUID registers unavailable\n", stderr); |
| return 1; |
| } |
| |
| get_cpu_ftr(ID_AA64ISAR0_EL1); |
| get_cpu_ftr(ID_AA64ISAR1_EL1); |
| get_cpu_ftr(ID_AA64MMFR0_EL1); |
| get_cpu_ftr(ID_AA64MMFR1_EL1); |
| get_cpu_ftr(ID_AA64PFR0_EL1); |
| get_cpu_ftr(ID_AA64PFR1_EL1); |
| get_cpu_ftr(ID_AA64DFR0_EL1); |
| get_cpu_ftr(ID_AA64DFR1_EL1); |
| |
| get_cpu_ftr(MIDR_EL1); |
| get_cpu_ftr(MPIDR_EL1); |
| get_cpu_ftr(REVIDR_EL1); |
| |
| #if 0 |
| /* Unexposed register access causes SIGILL */ |
| get_cpu_ftr(ID_MMFR0_EL1); |
| #endif |
| |
| return 0; |
| } |
