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

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Message-ID: <4BE06642.6080702@zytor.com>
Date:	Tue, 04 May 2010 11:24:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Avi Kivity <avi@...hat.com>, Brian Gerst <brgerst@...il.com>,
	"Cui, Dexuan" <dexuan.cui@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86: eliminate TS_XSAVE

On 05/04/2010 11:15 AM, Suresh Siddha wrote:
> On Tue, 2010-05-04 at 00:41 -0700, Avi Kivity wrote:
>> On 05/04/2010 12:45 AM, H. Peter Anvin wrote:
>>>
>>>> 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.
>>>
>>>    
>>
>> Ok, so you're happy with the patch as is?
> 
> As use_xsave() is in the hot context switch path, I would like to go
> with Avi's proposal.
> 

I would tend to agree.  Saving a likely cache miss in the hot context
switch path is worthwhile.

I would like to request one change, however.  I would like to see the
alternatives code to be:

	movb $0,reg
	movb $1,reg

... instead of using xor (which has to be padded with NOPs, which is of
course pointless since the slot is a fixed size.)  I would suggest using
a byte-sized variable instead of a dword-size variable to save a few
bytes, too.

Once the jump label framework is integrated and has matured, I think we
should consider using it to save the mov/test/jump.

	-hpa
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