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

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Date:   Thu, 8 Nov 2018 08:31:53 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>,
        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
Subject: Re: [RFC PATCH v1 1/2] x86/fpu: detect AVX task

On 2018/11/8 1:41, Tim Chen wrote:
> 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.

Right, thanks to point this out, will fix in v2 soon.

Thanks,
-Aubrey
> 
>> +			avx->decay_count--;
>> +		else
>> +			avx->state = 0;
>> +	}
>> +}
> 
> Tim
>