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

Avi Kivity
2012-04-09T13:20:47Z

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?

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