Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary

[Paolo Bonzini <pbonzini@redhat.com>]

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