linux-kernel - Re: [PATCH 4/6] KVM MMU: optimize for writing cr4

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Message-ID: <4BC41241.30204@redhat.com>
Date:	Tue, 13 Apr 2010 09:42:09 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM list <kvm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] KVM MMU: optimize for writing cr4

On 04/13/2010 06:07 AM, Xiao Guangrong wrote:
>
> And i found the commit 87778d60ee:
>
> |    KVM: MMU: Segregate mmu pages created with different cr4.pge settings
> |
> |    Don't allow a vcpu with cr4.pge cleared to use a shadow page created with
> |    cr4.pge set; this might cause a cr3 switch not to sync ptes that have the
> |    global bit set (the global bit has no effect if !cr4.pge).
> |
> |    This can only occur on smp with different cr4.pge settings for different
> |    vcpus (since a cr4 change will resync the shadow ptes), but there's no
> |    cost to being correct here.
>
> In current code, cr3 switch will sync all unsync shadow pages(regardless it's
> global or not) and this issue not live now, so, do we need also revert this
> patch?
>    

One path is to revert this patch.  The other is to restore the 
optimization that relies on it.  I'm not sure which is best.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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