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To:     Sean Christopherson <seanjc@google.com>,
        Marc Zyngier <maz@kernel.org>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
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        Julien Thierry <julien.thierry.kdev@gmail.com>,
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        Joerg Roedel <joro@8bytes.org>,
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        Ben Gardon <bgardon@google.com>
References: <20210402005658.3024832-1-seanjc@google.com>
 <20210402005658.3024832-10-seanjc@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v2 09/10] KVM: Don't take mmu_lock for range invalidation
 unless necessary
Message-ID: <417bd6b5-b7d0-ed22-adae-02150cdbfebe@redhat.com>
Date:   Fri, 2 Apr 2021 11:34:38 +0200
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On 02/04/21 02:56, Sean Christopherson wrote:
> Avoid taking mmu_lock for unrelated .invalidate_range_{start,end}()
> notifications.  Because mmu_notifier_count must be modified while holding
> mmu_lock for write, and must always be paired across start->end to stay
> balanced, lock elision must happen in both or none.  To meet that
> requirement, add a rwsem to prevent memslot updates across range_start()
> and range_end().
> 
> Use a rwsem instead of a rwlock since most notifiers _allow_ blocking,
> and the lock will be endl across the entire start() ... end() sequence.
> If anything in the sequence sleeps, including the caller or a different
> notifier, holding the spinlock would be disastrous.
> 
> For notifiers that _disallow_ blocking, e.g. OOM reaping, simply go down
> the slow path of unconditionally acquiring mmu_lock.  The sane
> alternative would be to try to acquire the lock and force the notifier
> to retry on failure.  But since OOM is currently the _only_ scenario
> where blocking is disallowed attempting to optimize a guest that has been
> marked for death is pointless.
> 
> Unconditionally define and use mmu_notifier_slots_lock in the memslots
> code, purely to avoid more #ifdefs.  The overhead of acquiring the lock
> is negligible when the lock is uncontested, which will always be the case
> when the MMU notifiers are not used.
> 
> Note, technically flag-only memslot updates could be allowed in parallel,
> but stalling a memslot update for a relatively short amount of time is
> not a scalability issue, and this is all more than complex enough.

Proposal for the locking documentation:

diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
index b21a34c34a21..3e4ad7de36cb 100644
--- a/Documentation/virt/kvm/locking.rst
+++ b/Documentation/virt/kvm/locking.rst
@@ -16,6 +16,13 @@ The acquisition orders for mutexes are as follows:
  - kvm->slots_lock is taken outside kvm->irq_lock, though acquiring
    them together is quite rare.
  
+- The kvm->mmu_notifier_slots_lock rwsem ensures that pairs of
+  invalidate_range_start() and invalidate_range_end() callbacks
+  use the same memslots array.  kvm->slots_lock is taken outside the
+  write-side critical section of kvm->mmu_notifier_slots_lock, so
+  MMU notifiers must not take kvm->slots_lock.  No other write-side
+  critical sections should be added.
+
  On x86, vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock.
  
  Everything else is a leaf: no other lock is taken inside the critical

Paolo