virtio: clear signalled_used_valid when switching from dataplane

When the dataplane thread stops, its vring.c implementation synchronizes
vring state back to virtio.c so we can continue emulating the virtio
device.

This patch ensures that virtio.c's signalled_used_valid flag is reset so
that we do not suppress guest notifications due to stale signalled_used
values.
Suggested-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/dataplane/vring.c

@@ -52,6 +52,7 @@ bool vring_setup(Vring *vring, VirtIODevice *vdev, int n)
 void vring_teardown(Vring *vring, VirtIODevice *vdev, int n)
 {
     virtio_queue_set_last_avail_idx(vdev, n, vring->last_avail_idx);
+    virtio_queue_invalidate_signalled_used(vdev, n);
 
     hostmem_finalize(&vring->hostmem);
 }

hw/virtio/virtio.c

@@ -1059,6 +1059,11 @@ void virtio_queue_set_last_avail_idx(VirtIODevice *vdev, int n, uint16_t idx)
     vdev->vq[n].last_avail_idx = idx;
 }
 
+void virtio_queue_invalidate_signalled_used(VirtIODevice *vdev, int n)
+{
+    vdev->vq[n].signalled_used_valid = false;
+}
+
 VirtQueue *virtio_get_queue(VirtIODevice *vdev, int n)
 {
     return vdev->vq + n;

include/hw/virtio/virtio.h

@@ -237,6 +237,7 @@ hwaddr virtio_queue_get_used_size(VirtIODevice *vdev, int n);
 hwaddr virtio_queue_get_ring_size(VirtIODevice *vdev, int n);
 uint16_t virtio_queue_get_last_avail_idx(VirtIODevice *vdev, int n);
 void virtio_queue_set_last_avail_idx(VirtIODevice *vdev, int n, uint16_t idx);
+void virtio_queue_invalidate_signalled_used(VirtIODevice *vdev, int n);
 VirtQueue *virtio_get_queue(VirtIODevice *vdev, int n);
 uint16_t virtio_get_queue_index(VirtQueue *vq);
 int virtio_queue_get_id(VirtQueue *vq);