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, 14 insertions, 0 deletions

diff --git a/balloon.c b/balloon.c
index aa354f7554..e02ab1c884 100644
--- a/balloon.c
+++ b/balloon.c
@@ -30,6 +30,7 @@
 #include "balloon.h"
 #include "trace.h"
 #include "qmp-commands.h"
+#include "qjson.h"
 
 static QEMUBalloonEvent *balloon_event_fn;
 static QEMUBalloonStatus *balloon_stat_fn;
@@ -80,6 +81,19 @@ static int qemu_balloon_status(BalloonInfo *info)
     return 1;
 }
 
+void qemu_balloon_changed(int64_t actual)
+{
+    QObject *data;
+
+    data = qobject_from_jsonf("{ 'actual': %" PRId64 " }",
+                              actual);
+
+    monitor_protocol_event(QEVENT_BALLOON_CHANGE, data);
+
+    qobject_decref(data);
+}
+
+
 BalloonInfo *qmp_query_balloon(Error **errp)
 {
     BalloonInfo *info;