Re: [PATCH v4 1/2] x86/fpu: Add a helper to prepare AMX state for low-power CPU idle
From: Dave Hansen
Date: Wed May 18 2022 - 11:41:31 EST
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.