linux-kernel - RE: [v6] kvm/fpu: Enable fully eager restore kvm FPU

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Date:	Fri, 24 Apr 2015 01:16:08 +0000
From:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>,
	"Hansen, Dave" <dave.hansen@...el.com>,
	"Li, Liang Z" <liang.z.li@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	"gleb@...nel.org" <gleb@...nel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"mtosatt@...hat.com" <mtosatt@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"joro@...tes.org" <joro@...tes.org>,
	"Hao, Xudong" <xudong.hao@...el.com>
Subject: RE: [v6] kvm/fpu: Enable fully eager restore kvm FPU

H. Peter Anvin wrote on 2015-04-24:
> On 04/23/2015 08:28 AM, Dave Hansen wrote:
>> On 04/23/2015 02:13 PM, Liang Li wrote:
>>> When compiling kernel on westmere, the performance of eager FPU is
>>> about 0.4% faster than lazy FPU.
>> 
>> Do you have an theory why this is?  What does the regression come from?
>> 
> 
> This is interesting since previous measurements on KVM have had the
> exact opposite results.  I think we need to understand this a lot more.

What I can tell is that vmexit is heavy. So it is reasonable to see the improvement under some cases, especially kernel is using eager FPU now which means each schedule may trigger a vmexit.

> 
> 	-hpa
>


Best regards,
Yang


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