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SubjectRe: [PATCH v4 1/2] x86/fpu: Add a helper to prepare AMX state for low-power CPU idle
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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(&current->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(&current->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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