virtio-blk: fix "disabled data plane" mode

In disabled mode, virtio-blk dataplane seems to be enabled, but flow
actually goes through the normal virtio path.  This patch simplifies a bit
the handling of disabled mode.  In disabled mode, virtio_blk_handle_output
might be called even if s->dataplane is not NULL.

This is a bit tricky, because the current check for s->dataplane will
always trigger, causing a continuous stream of calls to
virtio_blk_data_plane_start.  Unfortunately, these calls will not
do anything.  To fix this, set the "started" flag even in disabled
mode, and skip virtio_blk_data_plane_start if the started flag is true.
The resulting changes also prepare the code for the next patch, were
virtio-blk dataplane will reuse the same virtio_blk_handle_output function
as "regular" virtio-blk.

Because struct VirtIOBlockDataPlane is opaque in virtio-blk.c, we have
to move s->dataplane->started inside struct VirtIOBlock.

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>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/block/dataplane/virtio-blk.c

@@ -28,7 +28,6 @@
 #include "qom/object_interfaces.h"
 
 struct VirtIOBlockDataPlane {
-    bool started;
     bool starting;
     bool stopping;
     bool disabled;
@@ -264,11 +263,7 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
     VirtQueue *vq;
     int r;
 
-    if (s->started || s->disabled) {
-        return;
-    }
-
-    if (s->starting) {
+    if (vblk->dataplane_started || s->starting) {
         return;
     }
 
@@ -300,7 +295,7 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
     vblk->complete_request = complete_request_vring;
 
     s->starting = false;
-    s->started = true;
+    vblk->dataplane_started = true;
     trace_virtio_blk_data_plane_start(s);
 
     blk_set_aio_context(s->conf->conf.blk, s->ctx);
@@ -319,9 +314,10 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
     k->set_guest_notifiers(qbus->parent, 1, false);
   fail_guest_notifiers:
     vring_teardown(&s->vring, s->vdev, 0);
-    s->disabled = true;
   fail_vring:
+    s->disabled = true;
     s->starting = false;
+    vblk->dataplane_started = true;
 }
 
 /* Context: QEMU global mutex held */
@@ -331,13 +327,14 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
     VirtIOBlock *vblk = VIRTIO_BLK(s->vdev);
 
+    if (!vblk->dataplane_started || s->stopping) {
+        return;
+    }
 
     /* Better luck next time. */
     if (s->disabled) {
         s->disabled = false;
-        return;
-    }
-    if (!s->started || s->stopping) {
+        vblk->dataplane_started = false;
         return;
     }
     s->stopping = true;
@@ -364,6 +361,6 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
     /* Clean up guest notifier (irq) */
     k->set_guest_notifiers(qbus->parent, 1, false);
 
-    s->started = false;
+    vblk->dataplane_started = false;
     s->stopping = false;
 }

hw/block/virtio-blk.c

@@ -589,7 +589,7 @@ static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq)
     /* Some guests kick before setting VIRTIO_CONFIG_S_DRIVER_OK so start
      * dataplane here instead of waiting for .set_status().
      */
-    if (s->dataplane) {
+    if (s->dataplane && !s->dataplane_started) {
         virtio_blk_data_plane_start(s->dataplane);
         return;
     }

include/hw/virtio/virtio-blk.h

@@ -56,6 +56,7 @@ typedef struct VirtIOBlock {
     /* Function to push to vq and notify guest */
     void (*complete_request)(struct VirtIOBlockReq *req, unsigned char status);
     Notifier migration_state_notifier;
+    bool dataplane_started;
     struct VirtIOBlockDataPlane *dataplane;
 } VirtIOBlock;