linux-kernel - Re: [patch 2/3] x86/fpu/xsave: Prepare for optimized compaction

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Date:   Tue, 19 Apr 2022 15:33:31 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Andrew Cooper <andrew.cooper3@...rix.com>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Subject: Re: [patch 2/3] x86/fpu/xsave: Prepare for optimized compaction

On Tue, Apr 19 2022 at 14:39, Thomas Gleixner wrote:
> On Thu, Apr 14 2022 at 08:46, Dave Hansen wrote:
>>> +	for (i = fls64(xtmp) - 1; i >= FIRST_EXTENDED_XFEATURE;
>>> +	     i = fls64(xtmp) - 1) {
>>> +		unsigned int to, from;
>>
>> Is it worth a check here like:
>>
>> 		/* Do not move features in their init state: */
>> 		if (!(xcur & BIT_ULL(i))) {
>> 			xtmp &= ~BIT_ULL(i);
>> 			continue;
>> 		}
>
> That would also require to clear the bit in xall, but we can't do that
> in the loop as that affects offsets. Let me think about that.

Duh, misread it. Yes it's possible to do that. OTOH, with the optimized
compaction there won't be a bit set in xbv, but not in xfeatures, except
for the initial state when a task starts.

Thanks,

        tglx