linux-kernel - Re: [PATCH v5 6/9] KVM: MMU: introduce pte_prefetch_topup_memory

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Message-ID: <4C3BFB25.2010602@redhat.com>
Date:	Tue, 13 Jul 2010 08:35:33 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH v5 6/9] KVM: MMU: introduce pte_prefetch_topup_memory_cache()

On 07/13/2010 07:25 AM, Xiao Guangrong wrote:
>
>> Regressions? no.  Or do you mean the problem already exists?  Where?
>>      
> I mean this is a exist problem, likes invlpg, pte-write and sync-page, there are
> speculative path that it's not real access, but marked dirty if pte is writable.
>    

Right.  We should fix those too.

Prefetch is much more worrying though, especially with ept.  If a guest 
is using just 1/8 of the pages, it can look to migration as if it's 
using 100% of the pages.  The impact can be pretty large.

In contrast, I'm not too worried about invlpg, as most times an access 
will follow a miss, and usually a write access if we set a writeable 
pte.  Not sure about sync-page.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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