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

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Message-ID: <5539F6EC.6080604@redhat.com>
Date:	Fri, 24 Apr 2015 09:55:24 +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 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.

Paolo
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