Re: [PATCH v2 09/10] KVM: Don't take mmu_lock for range invalidation unless necessary
From: Sean Christopherson <seanjc@google.com>
Date: 2021-04-02 14:59:26
Also in:
kvm, kvmarm, linux-arm-kernel, lkml
On Fri, Apr 02, 2021, Paolo Bonzini wrote:
On 02/04/21 02:56, Sean Christopherson wrote:quoted
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:
Argh, sorry! Looks great, I owe you.
quoted hunk ↗ jump to hunk
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 criticalPaolo