Messages in this thread | | | From | Paolo Bonzini <> | Subject | Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary | Date | Wed, 31 Mar 2021 10:35:33 +0200 |
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On 26/03/21 03:19, Sean Christopherson wrote: > + /* > + * Reset the lock used to prevent memslot updates between MMU notifier > + * range_start and range_end. At this point no more MMU notifiers will > + * run, but the lock could still be held if KVM's notifier was removed > + * between range_start and range_end. No threads can be waiting on the > + * lock as the last reference on KVM has been dropped. If the lock is > + * still held, freeing memslots will deadlock. > + */ > + init_rwsem(&kvm->mmu_notifier_slots_lock);
I was going to say that this is nasty, then I noticed that mmu_notifier_unregister uses SRCU to ensure completion of concurrent calls to the MMU notifier. So I guess it's fine, but it's better to point it out:
/* * At this point no more MMU notifiers will run and pending * calls to range_start have completed, but the lock would * still be held and never released if the MMU notifier was * removed between range_start and range_end. Since the last * reference to the struct kvm has been dropped, no threads can * be waiting on the lock, but we might still end up taking it * when freeing memslots in kvm_arch_destroy_vm. Reset the lock * to avoid deadlocks. */
That said, the easiest way to avoid this would be to always update mmu_notifier_count. I don't mind the rwsem, but at least I suggest that you split the patch in two---the first one keeping the mmu_notifier_count update unconditional, and the second one introducing the rwsem and the on_lock function kvm_inc_notifier_count. Please document the new lock in Documentation/virt/kvm/locking.rst too.
Also, related to the first part of the series, perhaps you could structure the series in a slightly different way:
1) introduce the HVA walking API in common code, complete with on_lock and patch 15, so that you can use on_lock to increase mmu_notifier_seq
2) then migrate all architectures including x86 to the new API
IOW, first half of patch 10 and all of patch 15; then the second half of patch 10; then patches 11-14.
> +#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER) > + down_write(&kvm->mmu_notifier_slots_lock); > +#endif > rcu_assign_pointer(kvm->memslots[as_id], slots); > +#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER) > + up_write(&kvm->mmu_notifier_slots_lock); > +#endif
Please do this unconditionally, the cost is minimal if the rwsem is not contended (as is the case if the architecture doesn't use MMU notifiers at all).
Paolo
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