[Qemu-devel] [PATCH 4/4] virtio-balloon: keep collecting stats on save/restore

[Roman Kagan]

Upon save/restore virtio-balloon stats acquisition stops.  The reason is
that the fact that the (only) virtqueue element is being used by QEMU is
not recorded anywhere on save, so 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.

So instead just make sure the element is pushed before save, leaving the
ball on the guest side.  For that, add vm state change handler to
virtio-ballon which would take care of pushing the element if there is
one.

Signed-off-by: Roman Kagan <address@hidden>
Cc: "Michael S. Tsirkin" <address@hidden>
---
 hw/virtio/virtio-balloon.c         | 27 ++++++++++++++++++++++-----
 include/hw/virtio/virtio-balloon.h |  1 +
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index b56fecd..66a926a 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -88,10 +88,19 @@ static void balloon_stats_change_timer(VirtIOBalloon *s, 
int64_t secs)
     timer_mod(s->stats_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) + secs * 
1000);
 }
 
+static void balloon_stats_push_elem(VirtIOBalloon *s)
+{
+    VirtIODevice *vdev = VIRTIO_DEVICE(s);
+
+    virtqueue_push(s->svq, s->stats_vq_elem, s->stats_vq_offset);
+    virtio_notify(vdev, s->svq);
+    g_free(s->stats_vq_elem);
+    s->stats_vq_elem = NULL;
+}
+
 static void balloon_stats_poll_cb(void *opaque)
 {
     VirtIOBalloon *s = opaque;
-    VirtIODevice *vdev = VIRTIO_DEVICE(s);
 
     if (!s->stats_vq_elem) {
         /* The guest hasn't sent the stats yet (either not enabled or we came
@@ -100,10 +109,7 @@ static void balloon_stats_poll_cb(void *opaque)
         return;
     }
 
-    virtqueue_push(s->svq, s->stats_vq_elem, s->stats_vq_offset);
-    virtio_notify(vdev, s->svq);
-    g_free(s->stats_vq_elem);
-    s->stats_vq_elem = NULL;
+    balloon_stats_push_elem(s);
 }
 
 static void balloon_stats_get_all(Object *obj, Visitor *v, const char *name,
@@ -411,6 +417,15 @@ static int virtio_balloon_load_device(VirtIODevice *vdev, 
QEMUFile *f,
     return 0;
 }
 
+static void balloon_vm_state_change(void *opaque, int running, RunState state)
+{
+    VirtIOBalloon *s = opaque;
+
+    if (!running && s->stats_vq_elem) {
+        balloon_stats_push_elem(s);
+    }
+}
+
 static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
 {
     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
@@ -433,6 +448,7 @@ static void virtio_balloon_device_realize(DeviceState *dev, 
Error **errp)
     s->dvq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
     s->svq = virtio_add_queue(vdev, 1, virtio_balloon_receive_stats);
 
+    s->change = qemu_add_vm_change_state_handler(balloon_vm_state_change, s);
     reset_stats(s);
 }
 
@@ -441,6 +457,7 @@ static void virtio_balloon_device_unrealize(DeviceState 
*dev, Error **errp)
     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
     VirtIOBalloon *s = VIRTIO_BALLOON(dev);
 
+    qemu_del_vm_change_state_handler(s->change);
     balloon_stats_destroy_timer(s);
     qemu_remove_balloon_handler(s);
     virtio_cleanup(vdev);
diff --git a/include/hw/virtio/virtio-balloon.h 
b/include/hw/virtio/virtio-balloon.h
index 1ea13bd..d72ff7f 100644
--- a/include/hw/virtio/virtio-balloon.h
+++ b/include/hw/virtio/virtio-balloon.h
@@ -43,6 +43,7 @@ typedef struct VirtIOBalloon {
     int64_t stats_last_update;
     int64_t stats_poll_interval;
     uint32_t host_features;
+    VMChangeStateEntry *change;
 } VirtIOBalloon;
 
 #endif
-- 
2.7.4