Add event notification for guest balloon changes

After setting a balloon target value, applications have to
continually poll 'query-balloon' to determine whether the
guest has reacted to this request. The virtio-balloon backend
knows exactly when the guest has reacted though, and thus it
is possible to emit a JSON event to tell the mgmt application
whenever the guest balloon changes.

This introduces a new 'qemu_balloon_changed()' API which is
to be called by balloon driver backends, whenever they have
a change in balloon value. This takes the 'actual' balloon
value, as would be found in the BalloonInfo struct.

The qemu_balloon_change API emits a JSON monitor event which
looks like:

  {"timestamp": {"seconds": 1337162462, "microseconds": 814521},
   "event": "BALLOON_CHANGE", "data": {"actual": 944766976}}

* balloon.c, balloon.h: Introduce qemu_balloon_changed() for
  emitting balloon change events on the monitor
* hw/virtio-balloon.c: Invoke qemu_balloon_changed() whenever
  the guest changes the balloon actual value
* monitor.c, monitor.h: Define QEVENT_BALLOON_CHANGE

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Acked-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/monitor.c b/monitor.c
index a3bc2c7253..75fd4cf5e1 100644
--- a/monitor.c
+++ b/monitor.c
@@ -443,6 +443,7 @@ static const char *monitor_event_names[] = {
     [QEVENT_DEVICE_TRAY_MOVED] = "DEVICE_TRAY_MOVED",
     [QEVENT_SUSPEND] = "SUSPEND",
     [QEVENT_WAKEUP] = "WAKEUP",
+    [QEVENT_BALLOON_CHANGE] = "BALLOON_CHANGE",
 };
 QEMU_BUILD_BUG_ON(ARRAY_SIZE(monitor_event_names) != QEVENT_MAX)