linux-kernel - Re: [PATCH 5/9] KVM: MMU: fask check write-protect for direct mmu

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Date:	Fri, 20 Jul 2012 08:09:08 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 5/9] KVM: MMU: fask check write-protect for direct mmu

On Fri, Jul 20, 2012 at 10:34:28AM +0800, Xiao Guangrong wrote:
> On 07/20/2012 08:39 AM, Marcelo Tosatti wrote:
> > On Tue, Jul 17, 2012 at 09:53:29PM +0800, Xiao Guangrong wrote:
> >> If it have no indirect shadow pages we need not protect any gfn,
> >> this is always true for direct mmu without nested
> >>
> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> > 
> > Xiao,
> > 
> > What is the motivation? Numbers please.
> > 
> 
> mmu_need_write_protect is the common path for both soft-mmu and
> hard-mmu, checking indirect_shadow_pages can skip hash-table walking
> for the case which is tdp is enabled without nested guest.

I mean motivation as observation that it is a bottleneck.

> I will post the Number after I do the performance test.
> 
> > In fact, what case was the original indirect_shadow_pages conditional in
> > kvm_mmu_pte_write optimizing again?
> > 
> 
> They are the different paths, mmu_need_write_protect is the real
> page fault path, and kvm_mmu_pte_write is caused by mmio emulation.

Sure. What i am asking is, what use case is the indirect_shadow_pages
optimizing? What scenario, what workload? 

See the "When to optimize" section of
http://en.wikipedia.org/wiki/Program_optimization.

Can't remember why indirect_shadow_pages was introduced in
kvm_mmu_pte_write.

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