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

From: Sean Christopherson <seanjc@google.com>
To: David Stevens <stevensd@chromium.org>
Cc: Wanpeng Li <wanpengli@tencent.com>,
	kvm@vger.kernel.org, David Hildenbrand <david@redhat.com>,
	linux-mips@vger.kernel.org, Paul Mackerras <paulus@ozlabs.org>,
	Claudio Imbrenda <imbrenda@linux.ibm.com>,
	kvmarm@lists.cs.columbia.edu,
	Janosch Frank <frankja@linux.ibm.com>,
	Marc Zyngier <maz@kernel.org>, Joerg Roedel <joro@8bytes.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	Jim Mattson <jmattson@google.com>,
	Cornelia Huck <cohuck@redhat.com>,
	open list <linux-kernel@vger.kernel.org>,
	James Morse <james.morse@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>
Subject: Re: [PATCH] KVM: x86/mmu: consider the hva in mmu_notifer retry
Date: Tue, 26 Jan 2021 16:04:33 -0800	[thread overview]
Message-ID: <YBCuEaxZu0MuD3MW@google.com> (raw)
In-Reply-To: <CAD=HUj5YMtSJY6ZO9TRXHDEfWRM1o3Lrm7nkz=G2VJ_oZ-c5mw@mail.gmail.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.

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