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

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Date:	Mon, 03 May 2010 14:45:43 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Avi Kivity <avi@...hat.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Brian Gerst <brgerst@...il.com>, Dexuan Cui <dexuan.cui@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH 1/2] x86: eliminate TS_XSAVE

On 05/02/2010 10:44 AM, Avi Kivity wrote:
> 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 asked Suresh to comment on this, since he wrote the original code.  He
did confirm that the intent was to avoid a global memory reference.

	-hpa

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