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[PATCH v3 7/7] arm64: docs: document AT_HWCAP2 and unused AT_HWCAP bits

Now that we have expanded into AT_HWCAP2 let's add some documentation
to describe its presence. We also document the unused top half of
AT_HWCAP which we always return as 0 for userspace interoperation.

Signed-off-by: Andrew Murray <>
 Documentation/arm64/elf_hwcaps.txt | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/Documentation/arm64/elf_hwcaps.txt b/Documentation/arm64/elf_hwcaps.txt
index c605757dd4db..6b3d4d334db7 100644
--- a/Documentation/arm64/elf_hwcaps.txt
+++ b/Documentation/arm64/elf_hwcaps.txt
@@ -13,9 +13,9 @@ 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.
+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)
@@ -198,3 +198,11 @@ HWCAP_PACG
     Functionality implied by ID_AA64ISAR1_EL1.GPA == 0b0001 or
     ID_AA64ISAR1_EL1.GPI == 0b0001, as described by
+4. Unused AT_HWCAP bits
+Each AT_HWCAP and AT_HWCAP2 entry provides for up to 32 hwcaps contained
+in bits [31:0]. For interoperation with userspace we guarantee that bit
+62 of AT_HWCAP will always be returned as 0.

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