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

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Message-ID: <5008C3B4.1070006@linux.vnet.ibm.com>
Date:	Fri, 20 Jul 2012 10:34:28 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.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 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 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.

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