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ReStructuredText
338 lines
14 KiB
ReStructuredText
===========================
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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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+------------------------------+---------+---------+
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| Name | bits | visible |
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+------------------------------+---------+---------+
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| RNDR | [63-60] | y |
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+------------------------------+---------+---------+
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| TS | [55-52] | y |
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+------------------------------+---------+---------+
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| FHM | [51-48] | y |
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+------------------------------+---------+---------+
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| DP | [47-44] | y |
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+------------------------------+---------+---------+
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| SM4 | [43-40] | y |
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+------------------------------+---------+---------+
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| SM3 | [39-36] | y |
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+------------------------------+---------+---------+
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| SHA3 | [35-32] | y |
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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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2) ID_AA64PFR0_EL1 - Processor Feature Register 0
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+------------------------------+---------+---------+
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| Name | bits | visible |
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+------------------------------+---------+---------+
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| DIT | [51-48] | y |
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+------------------------------+---------+---------+
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| SVE | [35-32] | y |
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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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+------------------------------+---------+---------+
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3) ID_AA64PFR1_EL1 - Processor Feature Register 1
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+------------------------------+---------+---------+
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| Name | bits | visible |
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+------------------------------+---------+---------+
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| MTE | [11-8] | y |
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+------------------------------+---------+---------+
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| SSBS | [7-4] | y |
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+------------------------------+---------+---------+
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| BT | [3-0] | y |
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+------------------------------+---------+---------+
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4) MIDR_EL1 - Main ID Register
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+------------------------------+---------+---------+
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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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+------------------------------+---------+---------+
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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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5) ID_AA64ISAR1_EL1 - Instruction set attribute register 1
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+------------------------------+---------+---------+
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| Name | bits | visible |
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+------------------------------+---------+---------+
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| I8MM | [55-52] | y |
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+------------------------------+---------+---------+
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| DGH | [51-48] | y |
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+------------------------------+---------+---------+
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| BF16 | [47-44] | y |
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+------------------------------+---------+---------+
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| SB | [39-36] | y |
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+------------------------------+---------+---------+
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| FRINTTS | [35-32] | y |
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+------------------------------+---------+---------+
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| GPI | [31-28] | y |
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+------------------------------+---------+---------+
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| GPA | [27-24] | y |
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+------------------------------+---------+---------+
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| LRCPC | [23-20] | y |
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+------------------------------+---------+---------+
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| FCMA | [19-16] | y |
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+------------------------------+---------+---------+
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| JSCVT | [15-12] | y |
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+------------------------------+---------+---------+
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| API | [11-8] | y |
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+------------------------------+---------+---------+
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| APA | [7-4] | y |
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+------------------------------+---------+---------+
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| DPB | [3-0] | y |
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+------------------------------+---------+---------+
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6) ID_AA64MMFR2_EL1 - Memory model feature register 2
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+------------------------------+---------+---------+
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| Name | bits | visible |
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+------------------------------+---------+---------+
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| AT | [35-32] | y |
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+------------------------------+---------+---------+
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7) ID_AA64ZFR0_EL1 - SVE feature ID register 0
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+------------------------------+---------+---------+
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| Name | bits | visible |
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+------------------------------+---------+---------+
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| F64MM | [59-56] | y |
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+------------------------------+---------+---------+
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| F32MM | [55-52] | y |
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+------------------------------+---------+---------+
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| I8MM | [47-44] | y |
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+------------------------------+---------+---------+
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| SM4 | [43-40] | y |
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+------------------------------+---------+---------+
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| SHA3 | [35-32] | y |
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+------------------------------+---------+---------+
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| BF16 | [23-20] | y |
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+------------------------------+---------+---------+
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| BitPerm | [19-16] | y |
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+------------------------------+---------+---------+
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| AES | [7-4] | y |
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+------------------------------+---------+---------+
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| SVEVer | [3-0] | y |
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+------------------------------+---------+---------+
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Appendix I: Example
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-------------------
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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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