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Re: [Qemu-devel] [PATCH] Add event notification for guest balloon change

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] Add event notification for guest balloon changes
Date: Wed, 16 May 2012 13:58:34 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

On 05/16/2012 01:42 PM, Luiz Capitulino wrote:
On Wed, 16 May 2012 11:10:47 +0100
"Daniel P. Berrange"<address@hidden>  wrote:

From: "Daniel P. Berrange"<address@hidden>

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_change()' 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}}

It's missing an entry in QMP/qmp-events.txt and I have a comment below,
but in general looks good.

Amit, would be good to get your ack.

I think it would be safer to limit this event to (1) only firing once target has been reached (2) firing if target is deviated from without a corresponding change in target.

Otherwise, a guest could just flood libvirt with events. This would queue memory in QEMU indefinitely as the events got queued up to potentially serving as a DoS against other guests.


Anthony LIguori

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

Signed-off-by: Daniel P. Berrange<address@hidden>
  balloon.c           |   14 ++++++++++++++
  balloon.h           |    2 ++
  hw/virtio-balloon.c |    5 +++++
  monitor.c           |    3 +++
  monitor.h           |    1 +
  5 files changed, 25 insertions(+), 0 deletions(-)

diff --git a/balloon.c b/balloon.c
index aa354f7..913862b 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_change(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;
diff --git a/balloon.h b/balloon.h
index b60fd5d..2ebac0d 100644
--- a/balloon.h
+++ b/balloon.h
@@ -24,4 +24,6 @@ int qemu_add_balloon_handler(QEMUBalloonEvent *event_func,
                             QEMUBalloonStatus *stat_func, void *opaque);
  void qemu_remove_balloon_handler(void *opaque);

+void qemu_balloon_change(int64_t actual);
diff --git a/hw/virtio-balloon.c b/hw/virtio-balloon.c
index ce9d2c9..9137573 100644
--- a/hw/virtio-balloon.c
+++ b/hw/virtio-balloon.c
@@ -146,8 +146,13 @@ static void virtio_balloon_set_config(VirtIODevice *vdev,
      VirtIOBalloon *dev = to_virtio_balloon(vdev);
      struct virtio_balloon_config config;
+    uint32_t oldactual = dev->actual;
      memcpy(&config, config_data, 8);
      dev->actual = le32_to_cpu(config.actual);
+    if (dev->actual != oldactual) {
+        qemu_balloon_change(ram_size -
+                            (dev->actual<<  VIRTIO_BALLOON_PFN_SHIFT));
+    }

This can cause several events to be emitted until the memory is adjusted
to the value asked by the user. I'm undecided if this is a feature, but
if I were a client issuing the balloon command I'd expect to get the event
only when the memory is fully adjusted to the value I asked.

Not sure if this possible to implement though, or if we really want it.


  static uint32_t virtio_balloon_get_features(VirtIODevice *vdev, uint32_t f)
diff --git a/monitor.c b/monitor.c
index 12a6fe2..ef59cd9 100644
--- a/monitor.c
+++ b/monitor.c
@@ -493,6 +493,9 @@ void monitor_protocol_event(MonitorEvent event, QObject 
          case QEVENT_WAKEUP:
              event_name = "WAKEUP";
+            event_name = "BALLOON_CHANGE";
+            break;
diff --git a/monitor.h b/monitor.h
index 0d49800..8de0160 100644
--- a/monitor.h
+++ b/monitor.h
@@ -41,6 +41,7 @@ typedef enum MonitorEvent {
  } MonitorEvent;

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