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

[Yan Zhao]

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 :)