Add HWCAP2 support for AArch64 Linux. #1335
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Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Amanieu (or someone else) soon. Please see the contribution instructions for more information. |
HWCAP2 gets added on-demand for each architecture as they overflow HWCAP, and for recent users (such as AArch64), we are likely to encounter old kernels that do not provide HWCAP2. A present-but-zero value is also likely in practice, since HWCAP2 naturally contains recent architectural features. `getauxval(AT_HWCAP2)` cannot distinguish between absent and zero values, but to user-space Rust the meaning is the same: the corresponding features are not supported.
This doesn't extend the set of recognised features, but corrects some existing ones to refer to HWCAP2.
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I'm going to force-push to drop those Gerrit "Change-Id" tags. I apologise for the disruption. |
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Fixing the feature detection enabled a test that's apparently broken. (I didn't see it here because my hardware genuinely doesn't have that feature, so the test didn't run at all.) I'm investigating! |
These weren't run before because "i8mm" feature detection was broken. This change provides correct expected results, and also adds some negative inputs so that each variant behaves differently.
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This doesn't extend the set of recognised features, but corrects some existing ones to refer to
HWCAP2.Note that this changes the behaviour when
HWCAP2is absent on any platform. In particular:getauxval()could just mean that none of those features are enabled.HWCAP2is known to be truly absent (e.g. from reading/proc/self/auxv), but we haveHWCAP, then it's likely that the kernel is too old to support the corresponding features, and looking at a fallback isn't likely to be productive.