Re: [PATCH v4 1/2] x86/fpu: Add a helper to prepare AMX state for low-power CPU idle

From: Chang S. Bae
Date: Wed May 18 2022 - 13:46:39 EST

On 5/18/2022 8:41 AM, Dave Hansen wrote:

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);
}

With the note, I guess people will have no problem with AMX->XGETBV1. But I would leave this question to others who can tell.


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.

Yes, but FWIW, as it is non-architectural, the function should be consumed only by drivers for AMX systems.


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.

Yes, as long as AMX is wanted as a dynamic feature I think.

Thanks,
Chang