Messages in this thread | | | Date | Wed, 18 May 2022 08:41:12 -0700 | Subject | Re: [PATCH v4 1/2] x86/fpu: Add a helper to prepare AMX state for low-power CPU idle | From | Dave Hansen <> |
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On 5/17/22 15:24, Chang S. Bae wrote: > +/* > + * Initialize register state that may prevent from entering low-power idle. > + * This function will be invoked from the cpuidle driver only when needed. > + */ > +void fpu_idle_fpregs(void) > +{ > + if (cpu_feature_enabled(X86_FEATURE_XGETBV1) && > + (xfeatures_in_use() & XFEATURE_MASK_XTILE)) { > + tile_release(); > + fpregs_deactivate(¤t->thread.fpu); > + } > +}
This is a pretty minor nit, but:
X86_FEATURE_XFD depends on X86_FEATURE_XGETBV1
and
X86_FEATURE_AMX_TILE depends on X86_FEATURE_XFD
via cpu_deps[]. So there is an implicit dependency all the way from AMX to XGETBV1. It's also not patently obvious what X86_FEATURE_XGETBV1 has to do with the rest of the if().
Would this make more logical sense to folks?
/* Note: AMX_TILE being enabled implies XGETBV1 support */ if (cpu_feature_enabled(X86_FEATURE_AMX_TILE) && (xfeatures_in_use() & XFEATURE_MASK_XTILE)) { tile_release(); fpregs_deactivate(¤t->thread.fpu); }
That also has a nice side effect that non-AMX systems will get to use a static branch and can also skip over the XGETBV1 entirely.
The downside is that there's no explicit XGETBV1 check before calling xfeatures_in_use(). But, I don't really expect the AMX->XGETBV1 dependency to go away either.
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