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

Xiao Guangrong
2012-04-09T13:59:34Z

On 04/09/2012 09:20 PM, Avi Kivity wrote:

> 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?
>

Yes.

The gfn of direct sp is stable since it is calculated by sp->gfn which is
independent with gpte.