1. Introduction¶

Some hardware or software features are only available on some CPUimplementations, and/or with certain kernel configurations, but have noarchitected discovery mechanism available to userspace code at EL0. Thekernel exposes the presence of these features to userspace through a setof flags called hwcaps, exposed in the auxilliary vector.

Userspace software can test for features by acquiring the AT_HWCAP orAT_HWCAP2 entry of the auxiliary vector, and testing whether the relevantflags 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 checkthe relevant hwcap flag to verify that the feature is present beforeattempting to make use of the feature.

Features cannot be probed reliably through other means. When a featureis not available, attempting to use it may result in unpredictablebehaviour, and is not guaranteed to result in any reliable indicationthat the feature is unavailable, such as a SIGILL.

2. Interpretation of hwcaps¶

The majority of hwcaps are intended to indicate the presence of featureswhich are described by architected ID registers inaccessible touserspace code at EL0. These hwcaps are defined in terms of ID registerfields, and should be interpreted with reference to the definition ofthese 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 ARMdefines as being present when idreg.field has value val, but do notindicate that idreg.field is precisely equal to val, nor do theyindicate the absence of functionality implied by other values ofidreg.field.

Other hwcaps may indicate the presence of features which cannot bedescribed by ID registers alone. These may be described withoutreference 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 ofapproximately 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 extentdescribed 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 orID_AA64ISAR1_EL1.API == 0b0001, as described byDocumentation/arm64/pointer-authentication.rst.

HWCAP_PACG

Functionality implied by ID_AA64ISAR1_EL1.GPA == 0b0001 orID_AA64ISAR1_EL1.GPI == 0b0001, as described byDocumentation/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.

HWCAP2_BTI

Functionality implied by ID_AA64PFR0_EL1.BT == 0b0001.

4. Unused AT_HWCAP bits¶

For interoperation with userspace, the kernel guarantees that bits 62and 63 of AT_HWCAP will always be returned as 0.