On Tue, Jan 26, 2021, David Stevens wrote:

quoted

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.

quoted

+     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.

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