linux-kernel - Re: [PATCH] KVM: x86/mmu: consider the hva in mmu

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Date:   Tue, 26 Jan 2021 16:04:33 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Stevens <stevensd@...omium.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        linux-mips@...r.kernel.org, Paul Mackerras <paulus@...abs.org>,
        kvm-ppc@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Subject: Re: [PATCH] KVM: x86/mmu: consider the hva in mmu_notifer retry

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.