KVM: Assert that slots_lock is held when resetting per-vCPU dirty rings

Assert that slots_lock is held in kvm_dirty_ring_reset() and add a comment
to explain _why_ slots needs to be held for the duration of the reset.

Link: https://lore.kernel.org/all/[e-mail]
Suggested-by: James Houghton <[e-mail]>
Reviewed-by: Yan Zhao <[e-mail]>
Reviewed-by: Peter Xu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

virt/kvm/dirty_ring.c

···
 	unsigned long mask = 0;
 	struct kvm_dirty_gfn *entry;
 
+	/*
+	 * Ensure concurrent calls to KVM_RESET_DIRTY_RINGS are serialized,
+	 * e.g. so that KVM fully resets all entries processed by a given call
+	 * before returning to userspace.  Holding slots_lock also protects
+	 * the various memslot accesses.
+	 */
+	lockdep_assert_held(&kvm->slots_lock);
+
 	while (likely((*nr_entries_reset) < INT_MAX)) {
 		if (signal_pending(current))
 			return -EINTR;