linux-kernel - Re: [PATCH 00/13] KVM: MMU: fast page fault

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Message-ID: <4F82E228.7050602@redhat.com>
Date:	Mon, 09 Apr 2012 16:20:40 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC:	Xiao Guangrong <xiaoguangrong.eric@...il.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 00/13] KVM: MMU: fast page fault

On 04/06/2012 08:24 AM, Xiao Guangrong wrote:
>
> Foolish me, i should be crazy. Sorry for my mistake. :(
>
> Unfortunately, it can not work, we can not get a stable gfn from gpte or
> sp->gfns[]. For example:
>
> beginning:
> Gpte = Gfn1
> gfn_to_pfn(Gfn1) = Pfn
> Spte = Pfn
> Gfn1 is write-free
> Gfn2 is write-protected
>
>
> VCPU 0                              VCPU 1                     VCPU 2
>
> fault on gpte
> fast page fault path:
>   set Spte.fast_pf
>   get Gfn1 from Gpte/sp->gfns[]
>   if (Gfn1 is writable)
>                                 Pfn is swapped out:
> 					Spte = 0
> 				Gpte is modified to Gfn2,
>                                 and Pfn is realloced and remapped
>                                 to Gfn2, so:
>                                         Spte = Pfn
>
>                                                           fast page fault path:
>                                                              set Spte.fast_pf
>
>          cmpxchg  Spte+w
>             OOPS!!!
>   <we see Spte is not changed and
>    happily make it writable, so gfn2 can be writable>
>
> It seems only a unique identification can prevent this. :(
>

Ouch.

What about restricting this to role.direct=1?  Then gfn is stable?

-- 
error compiling committee.c: too many arguments to function

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