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

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Date:	Fri, 24 Apr 2015 08:08:06 +0000
From:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.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

Paolo Bonzini wrote on 2015-04-24:
> 
> 
> On 24/04/2015 09:46, Zhang, Yang Z wrote:
>>> 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).
>> 
>> 1000 cycles? I remember it takes about 4000 cycle even in HSW server.
> 
> I was going from memory, but I now measured it with the vmexit test of
> kvm-unit-tests.  With both SNB Xeon E5 and IVB Core i7, returns about
> 1400 clock cycles for a vmcall exit.  This includes the overhead of
> doing the cpuid itself.
> 
> Thus the vmexit cost is around 1300 cycles.  Of this the vmresume
> instruction is probably around 800 cycles, and the rest is introduced
> by KVM.  There are at least 4-5 memory barriers and locked instructions.

Yes, that's make sense. The average vmexit/vmentry handle cost is around 4000 cycles. But I guess xsaveopt doesn't take so many cycles. Does anyone have the xsaveopt cost data?

> 
> Paolo


Best regards,
Yang


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