KVM: Initialize irqfd waitqueue callback when adding to the queue

Initialize the irqfd waitqueue callback immediately prior to inserting the
irqfd into the eventfd's waitqueue.  Pre-initializing the state in a
completely different context is all kinds of confusing, and incorrectly
suggests that the waitqueue function needs to be initialize prior to
vfs_poll().

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250522235223.3178519-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

virt/kvm/eventfd.c

@@ -256,6 +256,13 @@ static void kvm_irqfd_register(struct file *file, wait_queue_head_t *wqh,
 	struct kvm_irqfd_pt *p = container_of(pt, struct kvm_irqfd_pt, pt);
 	struct kvm_kernel_irqfd *irqfd = p->irqfd;
 
+	/*
+	 * Add the irqfd as a priority waiter on the eventfd, with a custom
+	 * wake-up handler, so that KVM *and only KVM* is notified whenever the
+	 * underlying eventfd is signaled.
+	 */
+	init_waitqueue_func_entry(&irqfd->wait, irqfd_wakeup);
+
 	add_wait_queue_priority(wqh, &irqfd->wait);
 }
 
@@ -388,12 +395,6 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
 		mutex_unlock(&kvm->irqfds.resampler_lock);
 	}
 
-	/*
-	 * Install our own custom wake-up handling so we are notified via
-	 * a callback whenever someone signals the underlying eventfd
-	 */
-	init_waitqueue_func_entry(&irqfd->wait, irqfd_wakeup);
-
 	/*
 	 * Set the irqfd routing and add it to KVM's list before registering
 	 * the irqfd with the eventfd, so that the routing information is valid