linux-kernel - Re: [PATCH 7/7] KVM: MMU: don't walk every parent pages while mark unsync

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Date:	Tue, 15 Jun 2010 17:09:56 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH 7/7] KVM: MMU: don't walk every parent pages while mark
 unsync

On Tue, Jun 15, 2010 at 09:32:25AM +0800, Xiao Guangrong wrote:
> 
> 
> Marcelo Tosatti wrote:
> 
> >> -	if (!sp->multimapped) {
> >> -		kvm_mmu_update_unsync_bitmap(sp->parent_pte);
> >> +	if (sp->unsync_children++)
> >>  		return;
> > 
> > This looks wrong. If the sp has an unrelated children marked as 
> > unsync (which increased sp->unsync_children), you stop the walk? 
> > 
> 
> Marcelo,
> 
> I think it's right :-), we only walk the parents only when
> sp->unsync_children is 0, since sp->unsync_children is the number bit
> set in sp->unsync_child_bitmap, if sp->unsync_children > 0, we can sure
> its parents already have mark unsync-child-exist, assume, for example,
> have this mapping:
> 
>          / SP1
> P1 -> P2
>          \ SP2
> 
> [ P2 = P1.pte[0] SP1 = P2.pte[0] SP2 = P2.pte[1] ]
> 
> First, we mark SP1 unsyc, it will set:
> P2.unsync_child_bitmap[0] = 1, P2.unsync_children = 1
> and
> P1.unsync_child_bitmap[0] = 1, P1.unsync_children = 1
> 
> Then, we mark SP2 unsync, we only need do:
> P2.unsync_child_bitmap[1] = 1, P2.unsync_children = 2
> no need touch P1, since the P1 is already mark pte[0] unsync-child-exist.

You're right, applied.

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