linux-kernel - Re: [PATCH 11/11] KVM: MMU: improve write flooding detected

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Date:	Thu, 25 Aug 2011 15:57:22 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.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 11/11] KVM: MMU: improve write flooding detected

On 08/24/2011 03:09 AM, Marcelo Tosatti wrote:
> On Wed, Aug 24, 2011 at 12:32:32AM +0800, Xiao Guangrong wrote:
>> On 08/23/2011 08:38 PM, Marcelo Tosatti wrote:
>>
>>>> And, i think there are not problems since: if the spte without accssed bit is
>>>> written frequently, it means the guest page table is accessed infrequently or
>>>> during the writing, the guest page table is not accessed, in this time, zapping
>>>> this shadow page is not bad.
>>>
>>> Think of the following scenario:
>>>
>>> 1) page fault, spte with accessed bit is created from gpte at gfnA+indexA.
>>> 2) write to gfnA+indexA, spte has accessed bit set, write_flooding_count
>>> is not increased.
>>> 3) repeat
>>>
>>
>> I think the result is just we hoped, we do not want to zap the shadow page
>> because the spte is currently used by the guest, it also will be used in the
>> next repetition. So do not increase 'write_flooding_count' is a good choice.
> 
> Its not used. Step 2) is write to write protected shadow page at
> gfnA.
> 
>> Let's consider what will happen if we increase 'write_flooding_count':
>> 1: after three repetitions, zap the shadow page
>> 2: in step 1, we will alloc a new shadow page for gpte at gfnA+indexA
>> 3: in step 2, the flooding count is creased, so after 3 repetitions, the
>>    shadow page can be zapped again, repeat 1 to 3.
> 
> The shadow page will not be zapped because the spte created from
> gfnA+indexA has the accessed bit set:
> 
>        if (spte && !(*spte & shadow_accessed_mask))
>                sp->write_flooding_count++;
>        else
>                sp->write_flooding_count = 0;
> 

Marcelo, i am still confused with your example, in step 3), what is repeated?
it repeats step 2) or it repeats step 1) and 2)?

Only step 2) is repeated i guess, right? if it is yes, it works well:
when the guest writes gpte, the spte of corresponding shadow page is zapped
(level > 1) or it is speculatively fetched(level == 1), the accessed bit is
cleared in both case.

the later write can detect that the accessed bit is not set, and write_flooding_count
is increased. finally, the shadow page is zapped, the gpte is written directly.

>> The result is the shadow page for gfnA is alloced and zapped again and again,
>> yes?
> 
> The point is you cannot rely on the accessed bit of sptes that have been
> instantiated with the accessed bit set to decide whether or not to zap.
> Because the accessed bit will only be cleared on host memory pressure.
> 

But the accessed bit is also cleared after spte is written.
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