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

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Date:   Wed, 18 May 2022 08:41:12 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-pm@...r.kernel.org
Cc:     tglx@...utronix.de, dave.hansen@...ux.intel.com,
        peterz@...radead.org, bp@...en8.de, rafael@...nel.org,
        ravi.v.shankar@...el.com
Subject: Re: [PATCH v4 1/2] x86/fpu: Add a helper to prepare AMX state for
 low-power CPU idle

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.