dataplane: sync virtio.c and vring.c virtqueue state

Load the virtio.c state into vring.c when we start dataplane mode and
vice versa when stopping dataplane mode.  This patch makes it possible
to start and stop dataplane any time while the guest is running.

This will eventually allow us to go back to QEMU main loop for
bdrv_drain_all() and live migration.  In the meantime, this patch makes
the dataplane lifecycle more robust but should make no visible
difference.  It may be useful in the virtio-net dataplane effort.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/block/dataplane/virtio-blk.c

@@ -537,7 +537,7 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
     /* Clean up guest notifier (irq) */
     k->set_guest_notifiers(qbus->parent, 1, false);
 
-    vring_teardown(&s->vring);
+    vring_teardown(&s->vring, s->vdev, 0);
     s->started = false;
     s->stopping = false;
 }

hw/virtio/dataplane/vring.c

@@ -39,8 +39,8 @@ bool vring_setup(Vring *vring, VirtIODevice *vdev, int n)
 
     vring_init(&vring->vr, virtio_queue_get_num(vdev, n), vring_ptr, 4096);
 
-    vring->last_avail_idx = 0;
-    vring->last_used_idx = 0;
+    vring->last_avail_idx = virtio_queue_get_last_avail_idx(vdev, n);
+    vring->last_used_idx = vring->vr.used->idx;
     vring->signalled_used = 0;
     vring->signalled_used_valid = false;
 
@@ -49,8 +49,10 @@ bool vring_setup(Vring *vring, VirtIODevice *vdev, int n)
     return true;
 }
 
-void vring_teardown(Vring *vring)
+void vring_teardown(Vring *vring, VirtIODevice *vdev, int n)
 {
+    virtio_queue_set_last_avail_idx(vdev, n, vring->last_avail_idx);
+
     hostmem_finalize(&vring->hostmem);
 }
 

include/hw/virtio/dataplane/vring.h

@@ -50,7 +50,7 @@ static inline void vring_set_broken(Vring *vring)
 }
 
 bool vring_setup(Vring *vring, VirtIODevice *vdev, int n);
-void vring_teardown(Vring *vring);
+void vring_teardown(Vring *vring, VirtIODevice *vdev, int n);
 void vring_disable_notification(VirtIODevice *vdev, Vring *vring);
 bool vring_enable_notification(VirtIODevice *vdev, Vring *vring);
 bool vring_should_notify(VirtIODevice *vdev, Vring *vring);