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arm64: Documentation - Expose CPU feature registers
Documentation for the infrastructure to expose CPU feature register by emulating MRS. Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will.deacon@arm.com> Cc: Mark Rutland <mark.rutland@arm.com> Cc: Dave Martin <dave.martin@arm.com> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com> Signed-off-by: Will Deacon <will.deacon@arm.com>
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Suzuki K Poulose
Will Deacon
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ARM64 CPU Feature Registers
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===========================
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Author: Suzuki K Poulose <suzuki.poulose@arm.com>
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This file describes the ABI for exporting the AArch64 CPU ID/feature
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registers to userspace. The availability of this ABI is advertised
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via the HWCAP_CPUID in HWCAPs.
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1. Motivation
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---------------
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The ARM architecture defines a set of feature registers, which describe
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the capabilities of the CPU/system. Access to these system registers is
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restricted from EL0 and there is no reliable way for an application to
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extract this information to make better decisions at runtime. There is
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limited information available to the application via HWCAPs, however
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there are some issues with their usage.
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a) Any change to the HWCAPs requires an update to userspace (e.g libc)
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to detect the new changes, which can take a long time to appear in
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distributions. Exposing the registers allows applications to get the
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information without requiring updates to the toolchains.
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b) Access to HWCAPs is sometimes limited (e.g prior to libc, or
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when ld is initialised at startup time).
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c) HWCAPs cannot represent non-boolean information effectively. The
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architecture defines a canonical format for representing features
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in the ID registers; this is well defined and is capable of
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representing all valid architecture variations.
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2. Requirements
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-----------------
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a) Safety :
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Applications should be able to use the information provided by the
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infrastructure to run safely across the system. This has greater
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implications on a system with heterogeneous CPUs.
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The infrastructure exports a value that is safe across all the
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available CPU on the system.
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e.g, If at least one CPU doesn't implement CRC32 instructions, while
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others do, we should report that the CRC32 is not implemented.
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Otherwise an application could crash when scheduled on the CPU
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which doesn't support CRC32.
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b) Security :
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Applications should only be able to receive information that is
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relevant to the normal operation in userspace. Hence, some of the
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fields are masked out(i.e, made invisible) and their values are set to
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indicate the feature is 'not supported'. See Section 4 for the list
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of visible features. Also, the kernel may manipulate the fields
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based on what it supports. e.g, If FP is not supported by the
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kernel, the values could indicate that the FP is not available
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(even when the CPU provides it).
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c) Implementation Defined Features
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The infrastructure doesn't expose any register which is
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IMPLEMENTATION DEFINED as per ARMv8-A Architecture.
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d) CPU Identification :
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MIDR_EL1 is exposed to help identify the processor. On a
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heterogeneous system, this could be racy (just like getcpu()). The
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process could be migrated to another CPU by the time it uses the
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register value, unless the CPU affinity is set. Hence, there is no
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guarantee that the value reflects the processor that it is
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currently executing on. The REVIDR is not exposed due to this
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constraint, as REVIDR makes sense only in conjunction with the
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MIDR. Alternately, MIDR_EL1 and REVIDR_EL1 are exposed via sysfs
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at:
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/sys/devices/system/cpu/cpu$ID/regs/identification/
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\- midr
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\- revidr
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3. Implementation
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--------------------
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The infrastructure is built on the emulation of the 'MRS' instruction.
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Accessing a restricted system register from an application generates an
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exception and ends up in SIGILL being delivered to the process.
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The infrastructure hooks into the exception handler and emulates the
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operation if the source belongs to the supported system register space.
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The infrastructure emulates only the following system register space:
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Op0=3, Op1=0, CRn=0, CRm=0,4,5,6,7
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(See Table C5-6 'System instruction encodings for non-Debug System
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register accesses' in ARMv8 ARM DDI 0487A.h, for the list of
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registers).
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The following rules are applied to the value returned by the
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infrastructure:
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a) The value of an 'IMPLEMENTATION DEFINED' field is set to 0.
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b) The value of a reserved field is populated with the reserved
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value as defined by the architecture.
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c) The value of a 'visible' field holds the system wide safe value
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for the particular feature (except for MIDR_EL1, see section 4).
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d) All other fields (i.e, invisible fields) are set to indicate
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the feature is missing (as defined by the architecture).
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4. List of registers with visible features
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-------------------------------------------
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1) ID_AA64ISAR0_EL1 - Instruction Set Attribute Register 0
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x--------------------------------------------------x
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| Name | bits | visible |
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|--------------------------------------------------|
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| RES0 | [63-32] | n |
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|--------------------------------------------------|
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| RDM | [31-28] | y |
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|--------------------------------------------------|
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| ATOMICS | [23-20] | y |
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|--------------------------------------------------|
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| CRC32 | [19-16] | y |
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|--------------------------------------------------|
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| SHA2 | [15-12] | y |
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|--------------------------------------------------|
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| SHA1 | [11-8] | y |
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|--------------------------------------------------|
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| AES | [7-4] | y |
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|--------------------------------------------------|
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| RES0 | [3-0] | n |
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x--------------------------------------------------x
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2) ID_AA64PFR0_EL1 - Processor Feature Register 0
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x--------------------------------------------------x
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| Name | bits | visible |
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|--------------------------------------------------|
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| RES0 | [63-28] | n |
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|--------------------------------------------------|
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| GIC | [27-24] | n |
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|--------------------------------------------------|
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| AdvSIMD | [23-20] | y |
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|--------------------------------------------------|
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| FP | [19-16] | y |
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|--------------------------------------------------|
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| EL3 | [15-12] | n |
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|--------------------------------------------------|
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| EL2 | [11-8] | n |
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|--------------------------------------------------|
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| EL1 | [7-4] | n |
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|--------------------------------------------------|
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| EL0 | [3-0] | n |
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x--------------------------------------------------x
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3) MIDR_EL1 - Main ID Register
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x--------------------------------------------------x
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| Name | bits | visible |
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|--------------------------------------------------|
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| Implementer | [31-24] | y |
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|--------------------------------------------------|
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| Variant | [23-20] | y |
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|--------------------------------------------------|
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| Architecture | [19-16] | y |
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|--------------------------------------------------|
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| PartNum | [15-4] | y |
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|--------------------------------------------------|
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| Revision | [3-0] | y |
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x--------------------------------------------------x
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NOTE: The 'visible' fields of MIDR_EL1 will contain the value
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as available on the CPU where it is fetched and is not a system
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wide safe value.
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Appendix I: Example
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---------------------------
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/*
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* Sample program to demonstrate the MRS emulation ABI.
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*
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* Copyright (C) 2015-2016, ARM Ltd
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*
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* Author: Suzuki K Poulose <suzuki.poulose@arm.com>
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*/
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#include <asm/hwcap.h>
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#include <stdio.h>
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#include <sys/auxv.h>
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#define get_cpu_ftr(id) ({ \
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unsigned long __val; \
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asm("mrs %0, "#id : "=r" (__val)); \
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printf("%-20s: 0x%016lx\n", #id, __val); \
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})
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int main(void)
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{
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if (!(getauxval(AT_HWCAP) & HWCAP_CPUID)) {
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fputs("CPUID registers unavailable\n", stderr);
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return 1;
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}
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get_cpu_ftr(ID_AA64ISAR0_EL1);
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get_cpu_ftr(ID_AA64ISAR1_EL1);
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get_cpu_ftr(ID_AA64MMFR0_EL1);
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get_cpu_ftr(ID_AA64MMFR1_EL1);
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get_cpu_ftr(ID_AA64PFR0_EL1);
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get_cpu_ftr(ID_AA64PFR1_EL1);
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get_cpu_ftr(ID_AA64DFR0_EL1);
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get_cpu_ftr(ID_AA64DFR1_EL1);
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get_cpu_ftr(MIDR_EL1);
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get_cpu_ftr(MPIDR_EL1);
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get_cpu_ftr(REVIDR_EL1);
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#if 0
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/* Unexposed register access causes SIGILL */
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get_cpu_ftr(ID_MMFR0_EL1);
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#endif
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return 0;
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}
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@ -82,6 +82,10 @@ static bool __maybe_unused
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cpufeature_pan_not_uao(const struct arm64_cpu_capabilities *entry, int __unused);
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/*
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* NOTE: Any changes to the visibility of features should be kept in
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* sync with the documentation of the CPU feature register ABI.
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*/
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static const struct arm64_ftr_bits ftr_id_aa64isar0[] = {
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ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_EXACT, ID_AA64ISAR0_RDM_SHIFT, 4, 0),
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ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64ISAR0_ATOMICS_SHIFT, 4, 0),
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