185 lines
4.5 KiB
Plaintext
185 lines
4.5 KiB
Plaintext
ARM64 ELF hwcaps
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================
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This document describes the usage and semantics of the arm64 ELF hwcaps.
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1. Introduction
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---------------
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Some hardware or software features are only available on some CPU
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implementations, and/or with certain kernel configurations, but have no
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architected discovery mechanism available to userspace code at EL0. The
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kernel exposes the presence of these features to userspace through a set
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of flags called hwcaps, exposed in the auxilliary vector.
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Userspace software can test for features by acquiring the AT_HWCAP entry
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of the auxilliary vector, and testing whether the relevant flags are
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set, e.g.
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bool floating_point_is_present(void)
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{
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unsigned long hwcaps = getauxval(AT_HWCAP);
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if (hwcaps & HWCAP_FP)
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return true;
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return false;
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}
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Where software relies on a feature described by a hwcap, it should check
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the relevant hwcap flag to verify that the feature is present before
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attempting to make use of the feature.
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Features cannot be probed reliably through other means. When a feature
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is not available, attempting to use it may result in unpredictable
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behaviour, and is not guaranteed to result in any reliable indication
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that the feature is unavailable, such as a SIGILL.
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2. Interpretation of hwcaps
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---------------------------
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The majority of hwcaps are intended to indicate the presence of features
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which are described by architected ID registers inaccessible to
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userspace code at EL0. These hwcaps are defined in terms of ID register
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fields, and should be interpreted with reference to the definition of
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these fields in the ARM Architecture Reference Manual (ARM ARM).
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Such hwcaps are described below in the form:
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Functionality implied by idreg.field == val.
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Such hwcaps indicate the availability of functionality that the ARM ARM
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defines as being present when idreg.field has value val, but do not
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indicate that idreg.field is precisely equal to val, nor do they
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indicate the absence of functionality implied by other values of
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idreg.field.
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Other hwcaps may indicate the presence of features which cannot be
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described by ID registers alone. These may be described without
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reference to ID registers, and may refer to other documentation.
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3. The hwcaps exposed in AT_HWCAP
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---------------------------------
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HWCAP_FP
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Functionality implied by ID_AA64PFR0_EL1.FP == 0b0000.
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HWCAP_ASIMD
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Functionality implied by ID_AA64PFR0_EL1.AdvSIMD == 0b0000.
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HWCAP_EVTSTRM
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The generic timer is configured to generate events at a frequency of
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approximately 100KHz.
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HWCAP_AES
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Functionality implied by ID_AA64ISAR1_EL1.AES == 0b0001.
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HWCAP_PMULL
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Functionality implied by ID_AA64ISAR1_EL1.AES == 0b0010.
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HWCAP_SHA1
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Functionality implied by ID_AA64ISAR0_EL1.SHA1 == 0b0001.
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HWCAP_SHA2
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Functionality implied by ID_AA64ISAR0_EL1.SHA2 == 0b0001.
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HWCAP_CRC32
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Functionality implied by ID_AA64ISAR0_EL1.CRC32 == 0b0001.
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HWCAP_ATOMICS
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Functionality implied by ID_AA64ISAR0_EL1.Atomic == 0b0010.
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HWCAP_FPHP
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Functionality implied by ID_AA64PFR0_EL1.FP == 0b0001.
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HWCAP_ASIMDHP
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Functionality implied by ID_AA64PFR0_EL1.AdvSIMD == 0b0001.
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HWCAP_CPUID
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EL0 access to certain ID registers is available, to the extent
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described by Documentation/arm64/cpu-feature-registers.txt.
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These ID registers may imply the availability of features.
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HWCAP_ASIMDRDM
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Functionality implied by ID_AA64ISAR0_EL1.RDM == 0b0001.
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HWCAP_JSCVT
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Functionality implied by ID_AA64ISAR1_EL1.JSCVT == 0b0001.
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HWCAP_FCMA
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Functionality implied by ID_AA64ISAR1_EL1.FCMA == 0b0001.
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HWCAP_LRCPC
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Functionality implied by ID_AA64ISAR1_EL1.LRCPC == 0b0001.
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HWCAP_DCPOP
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Functionality implied by ID_AA64ISAR1_EL1.DPB == 0b0001.
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HWCAP_SHA3
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Functionality implied by ID_AA64ISAR0_EL1.SHA3 == 0b0001.
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HWCAP_SM3
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Functionality implied by ID_AA64ISAR0_EL1.SM3 == 0b0001.
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HWCAP_SM4
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Functionality implied by ID_AA64ISAR0_EL1.SM4 == 0b0001.
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HWCAP_ASIMDDP
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Functionality implied by ID_AA64ISAR0_EL1.DP == 0b0001.
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HWCAP_SHA512
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Functionality implied by ID_AA64ISAR0_EL1.SHA2 == 0b0002.
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HWCAP_SVE
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Functionality implied by ID_AA64PFR0_EL1.SVE == 0b0001.
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HWCAP_ASIMDFHM
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Functionality implied by ID_AA64ISAR0_EL1.FHM == 0b0001.
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HWCAP_DIT
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Functionality implied by ID_AA64PFR0_EL1.DIT == 0b0001.
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HWCAP_USCAT
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Functionality implied by ID_AA64MMFR2_EL1.AT == 0b0001.
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HWCAP_ILRCPC
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Functionality implied by ID_AA64ISR1_EL1.LRCPC == 0b0002.
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HWCAP_FLAGM
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Functionality implied by ID_AA64ISAR0_EL1.TS == 0b0001.
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HWCAP_SSBS
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Functionality implied by ID_AA64PFR1_EL1.SSBS == 0b0010.
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