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

[Sean Christopherson]

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.