Re: [PATCH 19/27] KVM: x86/mmu: Use page-track notifiers iff there are external users

Yan Zhao
2023-08-11T06:25:29Z

On Thu, Aug 10, 2023 at 08:41:14AM -0700, Sean Christopherson wrote:
> On Thu, Aug 10, 2023, Yan Zhao wrote:
> > On Thu, Aug 10, 2023 at 07:21:03AM +0800, Yan Zhao wrote:
> > > > Reading the value after acquiring mmu_lock ensures that both vCPUs will see whatever
> > > > value "loses" the race, i.e. whatever written value is processed second ('Y' in the
> > > > above sequence).
> > > I suspect that vCPU0 may still generate a wrong SPTE if vCPU1 wrote 4
> > > bytes while vCPU0 wrote 8 bytes, though the chances are very low.
> > >
> > This could happen in below sequence:
> > vCPU0 updates a PTE to AABBCCDD;
> > vCPU1 updates a PTE to EEFFGGHH in two writes.
> > (each character stands for a byte)
> >
> > vCPU0 vCPU1
> > write AABBCCDD
> > write GGHH
> > detect 4 bytes write and hold on sync
> > sync SPTE w/ AABBGGHH
> > write EEFF
> > sync SPTE w/ EEFFGGHH
> >
> >
> > Do you think it worth below serialization work?
>
> No, because I don't see any KVM bugs with the above sequence. If the guest doesn't
> ensure *all* writes from vCPU0 and vCPU1 are fully serialized, then it is completely
> legal for hardware (KVM in this case) to consume AABBGGHH as a PTE. The only thing
> the guest shouldn't see is EEFFCCDD, but I don't see how that can happen.
Ok, though still feel it's a little odd when a 1st cmpxch instruction on a GPA is still
under emulation, a 2nd or 3rd... cmpxch instruction to the same GPA may have returned
and they all succeeded :)