linux-kernel - Re: [PATCH v4 5/6] KVM: MMU: combine guest pte read between walk and pte prefetch

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Message-ID: <4C2F2393.9050301@redhat.com>
Date:	Sat, 03 Jul 2010 14:48:35 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH v4 5/6] KVM: MMU: combine guest pte read between walk
 and pte prefetch

On 07/02/2010 08:03 PM, Marcelo Tosatti wrote:
> On Thu, Jul 01, 2010 at 09:55:56PM +0800, Xiao Guangrong wrote:
>    
>> Combine guest pte read between guest pte walk and pte prefetch
>>
>> Signed-off-by: Xiao Guangrong<xiaoguangrong@...fujitsu.com>
>> ---
>>   arch/x86/kvm/paging_tmpl.h |   48 ++++++++++++++++++++++++++++++-------------
>>   1 files changed, 33 insertions(+), 15 deletions(-)
>>      
> Can't do this, it can miss invlpg:
>
> vcpu0			vcpu1
> read guest ptes
> 			modify guest pte
> 			invlpg
> instantiate stale
> guest pte
>
> See how the pte is reread inside fetch with mmu_lock held.
>    

Note, this is fine if the pte is unsync, since vcpu0 will soon invlpg 
it.  It's only broken for sync ptes.

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

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