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diff --git a/Documentation/arm64/elf_hwcaps.rst b/Documentation/arm64/elf_hwcaps.rst new file mode 100644 index 000000000000..c7cbf4b571c0 --- /dev/null +++ b/Documentation/arm64/elf_hwcaps.rst @@ -0,0 +1,201 @@ +================ +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. + +HWCAP2_DCPODP + + Functionality implied by ID_AA64ISAR1_EL1.DPB == 0b0010. + +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. + +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. + +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_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. + + +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. |