Re: [PATCH] KVM: x86/mmu: consider the hva in mmu_notifer retry

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 26, 2021, David Stevens wrote:
> > This needs a comment to explicitly state that 'count > 1' cannot be done at
> > this time.  My initial thought is that it would be more intuitive to check for
> > 'count > 1' here, but that would potentially check the wrong wrange when count
> > goes from 2->1.  The comment about persistence in invalidate_range_start() is a
> > good hint, but I think it's worth being explicit to avoid bad "cleanup" in the
> > future.
> >
> > > +     if (unlikely(kvm->mmu_notifier_count)) {
> > > +             if (kvm->mmu_notifier_range_start <= hva &&
> > > +                 hva < kvm->mmu_notifier_range_end)
> 
> I'm not sure I understand what you're suggesting here. How exactly
> would 'count > 1' be used incorrectly here? I'm fine with adding a
> comment, but I'm not sure what the comment needs to clarify.

There's no guarantee that the remaining in-progress invalidation when the count
goes from 2->1 is the same invalidation call that set range_start/range_end.

E.g. given two invalidations, A and B, the order of calls could be:

  kvm_mmu_notifier_invalidate_range_start(A)
  kvm_mmu_notifier_invalidate_range_start(B)
  kvm_mmu_notifier_invalidate_range_end(A)
  kvm_mmu_notifier_invalidate_range_end(B) <-- ???

or

  kvm_mmu_notifier_invalidate_range_start(A)
  kvm_mmu_notifier_invalidate_range_start(B)
  kvm_mmu_notifier_invalidate_range_end(B)
  kvm_mmu_notifier_invalidate_range_end(A) <-- ???

In the first case, "A" is in-progress when the count goes 2->1, in the second
case "B" is still in-progress.  Checking for "count > 1" in the consumer instead
of handling it in the producer (as you did) would lead to the consumer checking
against the wrong range.  I don't see a way to solve that without adding some
amount of history, which I agree is unnecessary.