linux-kernel - Re: [RFC PATCH v1 1/2] x86/fpu: detect AVX task

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Date:   Wed, 7 Nov 2018 09:41:05 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        mingo@...hat.com, peterz@...radead.org, hpa@...or.com
Cc:     ak@...ux.intel.com, arjan@...ux.intel.com,
        linux-kernel@...r.kernel.org, Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [RFC PATCH v1 1/2] x86/fpu: detect AVX task

On 11/06/2018 10:23 AM, Aubrey Li wrote:

> +static inline void update_avx_state(struct avx_state *avx)
> +{
> +	/*
> +	 * Check if XGETBV with ECX = 1 supported. XGETBV with ECX = 1
> +	 * returns the logical-AND of XCR0 and XINUSE. XINUSE is a bitmap
> +	 * by which the processor tracks the status of various components.
> +	 */
> +	if (!use_xgetbv1()) {
> +		avx->state = 0;
> +		return;
> +	}
> +	/*
> +	 * XINUSE is dynamic to track component state because VZEROUPPER
> +	 * happens on every function end and reset the bitmap to the
> +	 * initial configuration.
> +	 *
> +	 * State decay is introduced to solve the race condition between
> +	 * context switch and a function end. State is aggressively set
> +	 * once it's detected but need to be cleared by decay 3 context
> +	 * switches
> +	 */
> +	if (xgetbv(XINUSE_STATE_BITMAP_INDEX) & XFEATURE_MASK_Hi16_ZMM) {
> +		avx->state = 1;
> +		avx->decay_count = AVX_STATE_DECAY_COUNT;
> +	} else {
> +		if (!avx->decay_count)

Seems like the check should be

		if (avx->decay_count)

as we decrement the decay_count if it is non-zero.

> +			avx->decay_count--;
> +		else
> +			avx->state = 0;
> +	}
> +}

Tim