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-================
-ARM64 ELF hwcaps
-================
-
-This document describes the usage and semantics of the arm64 ELF hwcaps.
-
-
-1. Introduction
----------------
-
-Some hardware or software features are only available on some CPU
-implementations, and/or with certain kernel configurations, but have no
-architected discovery mechanism available to userspace code at EL0. The
-kernel exposes the presence of these features to userspace through a set
-of flags called hwcaps, exposed in the auxilliary vector.
-
-Userspace software can test for features by acquiring the AT_HWCAP or
-AT_HWCAP2 entry of the auxiliary vector, and testing whether the relevant
-flags are set, e.g.::
-
- bool floating_point_is_present(void)
- {
- unsigned long hwcaps = getauxval(AT_HWCAP);
- if (hwcaps & HWCAP_FP)
- return true;
-
- return false;
- }
-
-Where software relies on a feature described by a hwcap, it should check
-the relevant hwcap flag to verify that the feature is present before
-attempting to make use of the feature.
-
-Features cannot be probed reliably through other means. When a feature
-is not available, attempting to use it may result in unpredictable
-behaviour, and is not guaranteed to result in any reliable indication
-that the feature is unavailable, such as a SIGILL.
-
-
-2. Interpretation of hwcaps
----------------------------
-
-The majority of hwcaps are intended to indicate the presence of features
-which are described by architected ID registers inaccessible to
-userspace code at EL0. These hwcaps are defined in terms of ID register
-fields, and should be interpreted with reference to the definition of
-these fields in the ARM Architecture Reference Manual (ARM ARM).
-
-Such hwcaps are described below in the form::
-
- Functionality implied by idreg.field == val.
-
-Such hwcaps indicate the availability of functionality that the ARM ARM
-defines as being present when idreg.field has value val, but do not
-indicate that idreg.field is precisely equal to val, nor do they
-indicate the absence of functionality implied by other values of
-idreg.field.
-
-Other hwcaps may indicate the presence of features which cannot be
-described by ID registers alone. These may be described without
-reference to ID registers, and may refer to other documentation.
-
-
-3. The hwcaps exposed in AT_HWCAP
----------------------------------
-
-HWCAP_FP
- Functionality implied by ID_AA64PFR0_EL1.FP == 0b0000.
-
-HWCAP_ASIMD
- Functionality implied by ID_AA64PFR0_EL1.AdvSIMD == 0b0000.
-
-HWCAP_EVTSTRM
- The generic timer is configured to generate events at a frequency of
- approximately 100KHz.
-
-HWCAP_AES
- Functionality implied by ID_AA64ISAR0_EL1.AES == 0b0001.
-
-HWCAP_PMULL
- Functionality implied by ID_AA64ISAR0_EL1.AES == 0b0010.
-
-HWCAP_SHA1
- Functionality implied by ID_AA64ISAR0_EL1.SHA1 == 0b0001.
-
-HWCAP_SHA2
- Functionality implied by ID_AA64ISAR0_EL1.SHA2 == 0b0001.
-
-HWCAP_CRC32
- Functionality implied by ID_AA64ISAR0_EL1.CRC32 == 0b0001.
-
-HWCAP_ATOMICS
- Functionality implied by ID_AA64ISAR0_EL1.Atomic == 0b0010.
-
-HWCAP_FPHP
- Functionality implied by ID_AA64PFR0_EL1.FP == 0b0001.
-
-HWCAP_ASIMDHP
- Functionality implied by ID_AA64PFR0_EL1.AdvSIMD == 0b0001.
-
-HWCAP_CPUID
- EL0 access to certain ID registers is available, to the extent
- described by Documentation/arm64/cpu-feature-registers.rst.
-
- These ID registers may imply the availability of features.
-
-HWCAP_ASIMDRDM
- Functionality implied by ID_AA64ISAR0_EL1.RDM == 0b0001.
-
-HWCAP_JSCVT
- Functionality implied by ID_AA64ISAR1_EL1.JSCVT == 0b0001.
-
-HWCAP_FCMA
- Functionality implied by ID_AA64ISAR1_EL1.FCMA == 0b0001.
-
-HWCAP_LRCPC
- Functionality implied by ID_AA64ISAR1_EL1.LRCPC == 0b0001.
-
-HWCAP_DCPOP
- Functionality implied by ID_AA64ISAR1_EL1.DPB == 0b0001.
-
-HWCAP_SHA3
- Functionality implied by ID_AA64ISAR0_EL1.SHA3 == 0b0001.
-
-HWCAP_SM3
- Functionality implied by ID_AA64ISAR0_EL1.SM3 == 0b0001.
-
-HWCAP_SM4
- Functionality implied by ID_AA64ISAR0_EL1.SM4 == 0b0001.
-
-HWCAP_ASIMDDP
- Functionality implied by ID_AA64ISAR0_EL1.DP == 0b0001.
-
-HWCAP_SHA512
- Functionality implied by ID_AA64ISAR0_EL1.SHA2 == 0b0010.
-
-HWCAP_SVE
- Functionality implied by ID_AA64PFR0_EL1.SVE == 0b0001.
-
-HWCAP_ASIMDFHM
- Functionality implied by ID_AA64ISAR0_EL1.FHM == 0b0001.
-
-HWCAP_DIT
- Functionality implied by ID_AA64PFR0_EL1.DIT == 0b0001.
-
-HWCAP_USCAT
- Functionality implied by ID_AA64MMFR2_EL1.AT == 0b0001.
-
-HWCAP_ILRCPC
- Functionality implied by ID_AA64ISAR1_EL1.LRCPC == 0b0010.
-
-HWCAP_FLAGM
- Functionality implied by ID_AA64ISAR0_EL1.TS == 0b0001.
-
-HWCAP_SSBS
- Functionality implied by ID_AA64PFR1_EL1.SSBS == 0b0010.
-
-HWCAP_SB
- Functionality implied by ID_AA64ISAR1_EL1.SB == 0b0001.
-
-HWCAP_PACA
- Functionality implied by ID_AA64ISAR1_EL1.APA == 0b0001 or
- ID_AA64ISAR1_EL1.API == 0b0001, as described by
- Documentation/arm64/pointer-authentication.rst.
-
-HWCAP_PACG
- Functionality implied by ID_AA64ISAR1_EL1.GPA == 0b0001 or
- ID_AA64ISAR1_EL1.GPI == 0b0001, as described by
- Documentation/arm64/pointer-authentication.rst.
-
-HWCAP2_DCPODP
-
- Functionality implied by ID_AA64ISAR1_EL1.DPB == 0b0010.
-
-HWCAP2_SVE2
-
- Functionality implied by ID_AA64ZFR0_EL1.SVEVer == 0b0001.
-
-HWCAP2_SVEAES
-
- Functionality implied by ID_AA64ZFR0_EL1.AES == 0b0001.
-
-HWCAP2_SVEPMULL
-
- Functionality implied by ID_AA64ZFR0_EL1.AES == 0b0010.
-
-HWCAP2_SVEBITPERM
-
- Functionality implied by ID_AA64ZFR0_EL1.BitPerm == 0b0001.
-
-HWCAP2_SVESHA3
-
- Functionality implied by ID_AA64ZFR0_EL1.SHA3 == 0b0001.
-
-HWCAP2_SVESM4
-
- Functionality implied by ID_AA64ZFR0_EL1.SM4 == 0b0001.
-
-HWCAP2_FLAGM2
-
- Functionality implied by ID_AA64ISAR0_EL1.TS == 0b0010.
-
-HWCAP2_FRINT
-
- Functionality implied by ID_AA64ISAR1_EL1.FRINTTS == 0b0001.
-
-HWCAP2_SVEI8MM
-
- Functionality implied by ID_AA64ZFR0_EL1.I8MM == 0b0001.
-
-HWCAP2_SVEF32MM
-
- Functionality implied by ID_AA64ZFR0_EL1.F32MM == 0b0001.
-
-HWCAP2_SVEF64MM
-
- Functionality implied by ID_AA64ZFR0_EL1.F64MM == 0b0001.
-
-HWCAP2_SVEBF16
-
- Functionality implied by ID_AA64ZFR0_EL1.BF16 == 0b0001.
-
-HWCAP2_I8MM
-
- Functionality implied by ID_AA64ISAR1_EL1.I8MM == 0b0001.
-
-HWCAP2_BF16
-
- Functionality implied by ID_AA64ISAR1_EL1.BF16 == 0b0001.
-
-HWCAP2_DGH
-
- Functionality implied by ID_AA64ISAR1_EL1.DGH == 0b0001.
-
-HWCAP2_RNG
-
- Functionality implied by ID_AA64ISAR0_EL1.RNDR == 0b0001.
-
-4. Unused AT_HWCAP bits
------------------------
-
-For interoperation with userspace, the kernel guarantees that bits 62
-and 63 of AT_HWCAP will always be returned as 0.