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… > 0 Apple processors like A18/A19 physically support SME/SME2 in hardware, so hw.optional.arm.FEAT_SME returns true. However, XNU on iOS disables user-mode (EL0) SME execution (arm_sme_version() == 0), causing instructions like SMSTART to fault with EXC_BAD_INSTRUCTION. macOS and iPadOS (e.g. on M4) support and enable user-mode SME, setting hw.optional.arm.sme_max_svl_b > 0. Check hw.optional.arm.sme_max_svl_b > 0 before enabling SME feature flags to ensure SME is only reported when the OS allows user-space execution.
JakeStevens
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lemenkov
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Sep 19, 2026
Apple processors (such as A18/A19) may report
hw.optional.arm.FEAT_SME = 1 even when the XNU kernel on iOS disables
EL0 (user-mode) SME execution (hw.optional.arm.sme_max_svl_b == 0),
which causes SMSTART or RDSVL to fault with EXC_BAD_INSTRUCTION.
Gate SME/SME2 flags on have_feature("hw.optional.arm.sme_max_svl_b")
being nonzero. See pytorch/cpuinfo#432 and
pytorch/cpuinfo#433.
Also add SME2 detection test in cpu_test.cc and LIBYUV_DISABLE_SME2
environment variable support in unit_test.cc.
Test: libyuv_unittest --gtest_filter=*TestLinuxAArch64*
Bug: 8339302
Change-Id: If3e4d425ebaa89bf27965295ebdb43b844797ca1
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/8428226
Reviewed-by: Wan-Teh Chang <wtc@google.com>
Commit-Queue: Frank Barchard <fbarchard@google.com>
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Check hw.optional.arm.sme_max_svl_b > 0 before enabling SME feature flags to ensure SME is only reported when the OS allows user-space execution.
Fixes #432