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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 entry
+of the auxilliary 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_AA64ISAR1_EL1.AES == 0b0001.
+
+HWCAP_PMULL
+
+    Functionality implied by ID_AA64ISAR1_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.txt.
+
+    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 == 0b0002.
