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

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Date:	Fri, 24 Apr 2015 09:44:29 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Zhang, Yang Z" <yang.z.zhang@...el.com>,
	"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>,
	"mtosatti@...hat.com" <mtosatti@...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



On 24/04/2015 03:16, Zhang, Yang Z wrote:
>> 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.

On the other hand vmexit is lighter and lighter on newer processors; a
Sandy Bridge has less than half the vmexit cost of a Core 2 (IIRC 1000
vs. 2500 clock cycles approximately).

Also, measurement were done on Westmere but Sandy Bridge is the first
processor to have XSAVEOPT and thus use eager FPU.

Paolo
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