virtio-blk: always use dataplane path if ioeventfd is active

Override start_ioeventfd and stop_ioeventfd to start/stop the
whole dataplane logic.  This has some positive side effects:

- no need anymore for virtio_add_queue_aio (i.e. a revert of
  commit 0ff841f6d1)

- no need anymore to switch from generic ioeventfd handlers to
  dataplane

It detects some errors better:

    $ qemu-system-x86_64 -object iothread,id=io \
          -drive id=null,file=null-aio://,if=none,format=raw \
          -device virtio-blk-pci,ioeventfd=off,iothread=io,drive=null
    qemu-system-x86_64: -device virtio-blk-pci,ioeventfd=off,iothread=io,drive=null:
    ioeventfd is required for iothread

while previously it would have started just fine.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/block/dataplane/virtio-blk.c

@@ -88,23 +88,28 @@ void virtio_blk_data_plane_create(VirtIODevice *vdev, VirtIOBlkConf *conf,
 
     *dataplane = NULL;
 
-    if (!conf->iothread) {
-        return;
-    }
+    if (conf->iothread) {
+        if (!k->set_guest_notifiers || !k->ioeventfd_assign) {
+            error_setg(errp,
+                       "device is incompatible with iothread "
+                       "(transport does not support notifiers)");
+            return;
+        }
+        if (!virtio_device_ioeventfd_enabled(vdev)) {
+            error_setg(errp, "ioeventfd is required for iothread");
+            return;
+        }
 
+        /* If dataplane is (re-)enabled while the guest is running there could
+         * be block jobs that can conflict.
+         */
+        if (blk_op_is_blocked(conf->conf.blk, BLOCK_OP_TYPE_DATAPLANE, errp)) {
+            error_prepend(errp, "cannot start virtio-blk dataplane: ");
+            return;
+        }
+    }
     /* Don't try if transport does not support notifiers. */
-    if (!k->set_guest_notifiers || !k->ioeventfd_assign) {
-        error_setg(errp,
-                   "device is incompatible with dataplane "
-                   "(transport does not support notifiers)");
-        return;
-    }
-
-    /* If dataplane is (re-)enabled while the guest is running there could be
-     * block jobs that can conflict.
-     */
-    if (blk_op_is_blocked(conf->conf.blk, BLOCK_OP_TYPE_DATAPLANE, errp)) {
-        error_prepend(errp, "cannot start dataplane thread: ");
+    if (!virtio_device_ioeventfd_enabled(vdev)) {
         return;
     }
 
@@ -112,9 +117,13 @@ void virtio_blk_data_plane_create(VirtIODevice *vdev, VirtIOBlkConf *conf,
     s->vdev = vdev;
     s->conf = conf;
 
-    s->iothread = conf->iothread;
-    object_ref(OBJECT(s->iothread));
-    s->ctx = iothread_get_aio_context(s->iothread);
+    if (conf->iothread) {
+        s->iothread = conf->iothread;
+        object_ref(OBJECT(s->iothread));
+        s->ctx = iothread_get_aio_context(s->iothread);
+    } else {
+        s->ctx = qemu_get_aio_context();
+    }
     s->bh = aio_bh_new(s->ctx, notify_guest_bh, s);
     s->batch_notify_vqs = bitmap_new(conf->num_queues);
 
@@ -124,14 +133,19 @@ void virtio_blk_data_plane_create(VirtIODevice *vdev, VirtIOBlkConf *conf,
 /* Context: QEMU global mutex held */
 void virtio_blk_data_plane_destroy(VirtIOBlockDataPlane *s)
 {
+    VirtIOBlock *vblk;
+
     if (!s) {
         return;
     }
 
-    virtio_blk_data_plane_stop(s);
+    vblk = VIRTIO_BLK(s->vdev);
+    assert(!vblk->dataplane_started);
     g_free(s->batch_notify_vqs);
     qemu_bh_delete(s->bh);
-    object_unref(OBJECT(s->iothread));
+    if (s->iothread) {
+        object_unref(OBJECT(s->iothread));
+    }
     g_free(s);
 }
 
@@ -147,17 +161,18 @@ static void virtio_blk_data_plane_handle_output(VirtIODevice *vdev,
 }
 
 /* Context: QEMU global mutex held */
-void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
+int virtio_blk_data_plane_start(VirtIODevice *vdev)
 {
-    BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(s->vdev)));
+    VirtIOBlock *vblk = VIRTIO_BLK(vdev);
+    VirtIOBlockDataPlane *s = vblk->dataplane;
+    BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(vblk)));
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
-    VirtIOBlock *vblk = VIRTIO_BLK(s->vdev);
     unsigned i;
     unsigned nvqs = s->conf->num_queues;
     int r;
 
     if (vblk->dataplane_started || s->starting) {
-        return;
+        return 0;
     }
 
     s->starting = true;
@@ -204,20 +219,22 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
                 virtio_blk_data_plane_handle_output);
     }
     aio_context_release(s->ctx);
-    return;
+    return 0;
 
   fail_guest_notifiers:
     vblk->dataplane_disabled = true;
     s->starting = false;
     vblk->dataplane_started = true;
+    return -ENOSYS;
 }
 
 /* Context: QEMU global mutex held */
-void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
+void virtio_blk_data_plane_stop(VirtIODevice *vdev)
 {
-    BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(s->vdev)));
+    VirtIOBlock *vblk = VIRTIO_BLK(vdev);
+    VirtIOBlockDataPlane *s = vblk->dataplane;
+    BusState *qbus = qdev_get_parent_bus(DEVICE(vblk));
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
-    VirtIOBlock *vblk = VIRTIO_BLK(s->vdev);
     unsigned i;
     unsigned nvqs = s->conf->num_queues;