[Qemu-devel] [PATCH 2/2] virtio-balloon: resume collecting stats on vmload

[Roman Kagan <rkagan@virtuozzo.com>]

Upon save/restore virtio-balloon stats acquisition stops.  The reason is
that the in-use virtqueue element is not saved, and upon restore it's
not released to the guest, making further progress impossible.

Saving the information about the used element would introduce
unjustified vmstate incompatibility.

However, the number of virtqueue in-use elements can be deduced from the
data available on restore (see bccdef6b1a204db0f41ffb6e24ce373e4d7890d4
"virtio: recalculate vq->inuse after migration").

So make the stats virtqueue forget that an element was popped from it,
and start over.  As this tackles the problem on the "load" side, it
is compatible with the state saved by earlier QEMU versions.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Ladi Prosek <lprosek@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/virtio/virtio-balloon.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index 5af429a..8660052 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -406,7 +406,13 @@ static void virtio_balloon_save_device(VirtIODevice *vdev, QEMUFile *f)
 
 static int virtio_balloon_load(QEMUFile *f, void *opaque, size_t size)
 {
-    return virtio_load(VIRTIO_DEVICE(opaque), f, 1);
+    VirtIODevice *vdev = VIRTIO_DEVICE(opaque);
+    VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
+    int ret = virtio_load(vdev, f, 1);
+    /* rewind needs vq->inuse populated which happens in virtio_load() after
+     * vdev->load */
+    virtqueue_rewind(s->svq);
+    return ret;
 }
 
 static int virtio_balloon_load_device(VirtIODevice *vdev, QEMUFile *f,
@@ -468,6 +474,18 @@ static void virtio_balloon_device_reset(VirtIODevice *vdev)
     }
 }
 
+static void virtio_balloon_set_status(VirtIODevice *vdev, uint8_t status)
+{
+    VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
+
+    if (vdev->vm_running && balloon_stats_supported(s) &&
+        (status & VIRTIO_CONFIG_S_DRIVER_OK) && !s->stats_vq_elem) {
+        /* poll stats queue for the element we may have discarded when the VM
+         * was stopped */
+        virtio_balloon_receive_stats(vdev, s->svq);
+    }
+}
+
 static void virtio_balloon_instance_init(Object *obj)
 {
     VirtIOBalloon *s = VIRTIO_BALLOON(obj);
@@ -505,6 +523,7 @@ static void virtio_balloon_class_init(ObjectClass *klass, void *data)
     vdc->get_features = virtio_balloon_get_features;
     vdc->save = virtio_balloon_save_device;
     vdc->load = virtio_balloon_load_device;
+    vdc->set_status = virtio_balloon_set_status;
 }
 
 static const TypeInfo virtio_balloon_info = {
-- 
2.7.4