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

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Message-ID: <4CF54C4F.80901@cn.fujitsu.com>
Date:	Wed, 01 Dec 2010 03:11:11 +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 02:38 AM, Gleb Natapov wrote:

>> It can't avoid the page to be evicted again since the page is marked accessed only
>> when spte is droped or updated.
> I still do not understand why are you disabling prefault for ept. Why
> do you want to distinguish between actually accessed translations and
> prefauls? What problem are you trying to fix?
> 

Look at set_spte_track_bits() function:

	if (!shadow_accessed_mask || old_spte & shadow_accessed_mask)
		kvm_set_pfn_accessed(pfn);

It's always mark the page accessed.

But prefault is the speculative path, the prefault address may not be
accessed later(the apf process is killed). under this case, the page is
not really accessed.
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