virtio-blk: always set ioeventfd during startup

When starting ioeventfd it is common practice to set the event notifier
so that the ioeventfd handler is triggered to run immediately. There may
be no requests waiting to be processed, but the idea is that if a
request snuck in then we guarantee that it will be detected.

One scenario where self-triggering the ioeventfd is necessary is when
virtio_blk_handle_output() is called from a vCPU thread before the
VIRTIO Device Status transitions to DRIVER_OK. In that case we need to
self-trigger the ioeventfd so that the kick handled by the vCPU thread
causes the vq AioContext thread to take over handling the request(s).

Fixes: b6948ab ("virtio-blk: add iothread-vq-mapping parameter")
Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20240119135748.270944-7-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/block/virtio-blk.c

@@ -1809,14 +1809,14 @@ static int virtio_blk_start_ioeventfd(VirtIODevice *vdev)
     smp_wmb(); /* paired with aio_notify_accept() on the read side */
 
     /* Get this show started by hooking up our callbacks */
-    if (!blk_in_drain(s->conf.conf.blk)) {
-        for (i = 0; i < nvqs; i++) {
-            VirtQueue *vq = virtio_get_queue(vdev, i);
-            AioContext *ctx = s->vq_aio_context[i];
+    for (i = 0; i < nvqs; i++) {
+        VirtQueue *vq = virtio_get_queue(vdev, i);
+        AioContext *ctx = s->vq_aio_context[i];
 
-            /* Kick right away to begin processing requests already in vring */
-            event_notifier_set(virtio_queue_get_host_notifier(vq));
+        /* Kick right away to begin processing requests already in vring */
+        event_notifier_set(virtio_queue_get_host_notifier(vq));
 
+        if (!blk_in_drain(s->conf.conf.blk)) {
             virtio_queue_aio_attach_host_notifier(vq, ctx);
         }
     }