linux-kernel - Re: [PATCH 1/2] x86: eliminate TS

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Date:	Sun, 02 May 2010 20:44:56 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Brian Gerst <brgerst@...il.com>
CC:	Dexuan Cui <dexuan.cui@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 1/2] x86: eliminate TS_XSAVE

On 05/02/2010 08:38 PM, Brian Gerst wrote:
> On Sun, May 2, 2010 at 10:53 AM, Avi Kivity<avi@...hat.com>  wrote:
>    
>> The fpu code currently uses current->thread_info->status&  TS_XSAVE as
>> a way to distinguish between XSAVE capable processors and older processors.
>> The decision is not really task specific; instead we use the task status to
>> avoid a global memory reference - the value should be the same across all
>> threads.
>>
>> Eliminate this tie-in into the task structure by using an alternative
>> instruction keyed off the XSAVE cpu feature; this results in shorter and
>> faster code, without introducing a global memory reference.
>>      
> I think you should either just use cpu_has_xsave, or extend this use
> of alternatives to all cpu features.  It doesn't make sense to only do
> it for xsave.
>    

I was trying to avoid a performance regression relative to the current 
code, as it appears that some care was taken to avoid the memory reference.

I agree that it's probably negligible compared to the save/restore 
code.  If the x86 maintainers agree as well, I'll replace it with 
cpu_has_xsave.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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