virtio-scsi: Set host notifiers and callbacks separately

Host notifiers are guaranteed to be idle until the callbacks are
hooked up with virtio_queue_aio_set_host_notifier_handler(). They
thus don't need to be set or unset with the AioContext lock held.

Do this outside the critical section, like virtio-blk already
does : basically downgrading virtio_scsi_vring_init() to only
setup the host notifier and set the callback in the caller.

This will allow to batch addition/deletion of ioeventds in
a single memory transaction, which is expected to greatly
improve initialization time.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20210407143501.244343-4-groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/scsi/virtio-scsi-dataplane.c

@@ -94,8 +94,7 @@ static bool virtio_scsi_data_plane_handle_event(VirtIODevice *vdev,
     return progress;
 }
 
-static int virtio_scsi_vring_init(VirtIOSCSI *s, VirtQueue *vq, int n,
-                                  VirtIOHandleAIOOutput fn)
+static int virtio_scsi_set_host_notifier(VirtIOSCSI *s, VirtQueue *vq, int n)
 {
     BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(s)));
     int rc;
@@ -109,7 +108,6 @@ static int virtio_scsi_vring_init(VirtIOSCSI *s, VirtQueue *vq, int n,
         return rc;
     }
 
-    virtio_queue_aio_set_host_notifier_handler(vq, s->ctx, fn);
     return 0;
 }
 
@@ -154,38 +152,44 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
         goto fail_guest_notifiers;
     }
 
-    aio_context_acquire(s->ctx);
-    rc = virtio_scsi_vring_init(s, vs->ctrl_vq, 0,
-                                virtio_scsi_data_plane_handle_ctrl);
-    if (rc) {
-        goto fail_vrings;
+    rc = virtio_scsi_set_host_notifier(s, vs->ctrl_vq, 0);
+    if (rc != 0) {
+        goto fail_host_notifiers;
     }
 
     vq_init_count++;
-    rc = virtio_scsi_vring_init(s, vs->event_vq, 1,
-                                virtio_scsi_data_plane_handle_event);
-    if (rc) {
-        goto fail_vrings;
+    rc = virtio_scsi_set_host_notifier(s, vs->event_vq, 1);
+    if (rc != 0) {
+        goto fail_host_notifiers;
     }
 
     vq_init_count++;
+
     for (i = 0; i < vs->conf.num_queues; i++) {
-        rc = virtio_scsi_vring_init(s, vs->cmd_vqs[i], i + 2,
-                                    virtio_scsi_data_plane_handle_cmd);
+        rc = virtio_scsi_set_host_notifier(s, vs->cmd_vqs[i], i + 2);
         if (rc) {
-            goto fail_vrings;
+            goto fail_host_notifiers;
         }
         vq_init_count++;
     }
 
+    aio_context_acquire(s->ctx);
+    virtio_queue_aio_set_host_notifier_handler(vs->ctrl_vq, s->ctx,
+                                            virtio_scsi_data_plane_handle_ctrl);
+    virtio_queue_aio_set_host_notifier_handler(vs->event_vq, s->ctx,
+                                           virtio_scsi_data_plane_handle_event);
+
+    for (i = 0; i < vs->conf.num_queues; i++) {
+        virtio_queue_aio_set_host_notifier_handler(vs->cmd_vqs[i], s->ctx,
+                                             virtio_scsi_data_plane_handle_cmd);
+    }
+
     s->dataplane_starting = false;
     s->dataplane_started = true;
     aio_context_release(s->ctx);
     return 0;
 
-fail_vrings:
-    aio_wait_bh_oneshot(s->ctx, virtio_scsi_dataplane_stop_bh, s);
-    aio_context_release(s->ctx);
+fail_host_notifiers:
     for (i = 0; i < vq_init_count; i++) {
         virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
         virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i);