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

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Date:	Fri, 20 Jul 2012 21:33:58 +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 07:09 PM, Marcelo Tosatti wrote:
> 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? 
> 

Sorry, Marcelo, i do know why i completely misunderstood your mail. :(

I am not sure whether this is a bottleneck, i just got it from
code review, i will measure it to see if we can get benefit from
it. :p

> 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.
> 

Please refer to:
	https://lkml.org/lkml/2011/5/18/174

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