linux-kernel - Re: [PATCH v3 2/3] KVM: MMU: fix accessed bit set on prefault path

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Message-ID: <4CF53F41.1040503@cn.fujitsu.com>
Date:	Wed, 01 Dec 2010 02:15:29 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 2/3] KVM: MMU: fix accessed bit set on prefault path

On 12/01/2010 01:50 AM, Gleb Natapov wrote:
> On Wed, Dec 01, 2010 at 12:52:22AM +0800, Xiao Guangrong wrote:
>> On 11/30/2010 09:29 PM, Gleb Natapov wrote:
>>
>>>> +	if (!shadow_accessed_mask)
>>>> +		return;
>>>> +
>>> I don't get this. As far as I can see VMX inits shadow_accessed_mask to
>>> be zero if ept is enabled. This line here means that we never prefault with ept
>>> enabled. It is opposite from what it should be.
>>>
>>
>> Since it's no accessed bit on EPT, it's no way to distinguish between actually
>> accessed translations and prefault.
> Why is this a problem? We do what this page to not be evicted again
> since we expect it to be accessed.
> 

It can't avoid the page to be evicted again since the page is marked accessed only
when spte is droped or updated.
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