linux-kernel - Re: [PATCH v3 11/11] KVM: MMU: improve write flooding detected

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Message-ID: <4E6FACBD.1020801@cn.fujitsu.com>
Date:	Wed, 14 Sep 2011 03:19:25 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 11/11] KVM: MMU: improve write flooding detected

On 09/13/2011 07:07 PM, Avi Kivity wrote:
> On 08/30/2011 05:38 AM, Xiao Guangrong wrote:
>> Detecting write-flooding does not work well, when we handle page written, if
>> the last speculative spte is not accessed, we treat the page is
>> write-flooding, however, we can speculative spte on many path, such as pte
>> prefetch, page synced, that means the last speculative spte may be not point
>> to the written page and the written page can be accessed via other sptes, so
>> depends on the Accessed bit of the last speculative spte is not enough
>>
>> Instead of detected page accessed, we can detect whether the spte is accessed
>> after it is written, if the spte is not accessed but it is written frequently,
>> we treat is not a page table or it not used for a long time
>>
>>
> 
> The spte may not be accessed, but other sptes in the same page can be accessed.  An example is the fixmap area for kmap_atomic(), there will be a lot of pte writes but other sptes will be accessed without going through soft-mmu at all.

I think this kind of shadow pae is mostly the last page table(level=1), maybe
we can skip the write-flooding for the last shadow page, because the last shadow
page can become unsync and it can not let page table write-protected.
 
> I think you have to read the parent_ptes->spte.accessed bits to be sure.
> 

I guess the overload of this way is little high:
- it needs to walk parent ptes for every shadow pages
- we need to clear the parent_ptes->spte.accessed bit when the page is written, and
  the tlb flush is needed.
no?

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