linux-kernel - Re: [PATCH v2 1/2] x86/fpu: Add a helper to prepare AMX state for low-power CPU idle

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Date:   Wed, 9 Mar 2022 14:46:30 -0800
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 v2 1/2] x86/fpu: Add a helper to prepare AMX state for
 low-power CPU idle

On 3/9/22 14:34, 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 (!fpu_state_size_dynamic())
> +		return;

Is this check just an optimization?  I'm having trouble imagining a
situation where we would have:

	(xfeatures_in_use() & XFEATURE_MASK_XTILE) == true
but
	fpu_state_size_dynamic() == false

> +	if (xfeatures_in_use() & XFEATURE_MASK_XTILE) {
> +		tile_release();
> +		fpregs_deactivate(&current->thread.fpu);
> +	}
> +}

xfeatures_in_use() isn't exactly expensive either.