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

[Luiz Capitulino <lcapitulino@redhat.com>]

On Wed, 16 May 2012 11:10:47 +0100
"Daniel P. Berrange" <berrange@redhat.com> wrote:

> From: "Daniel P. Berrange" <berrange@redhat.com>
> 
> 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.

> 
> * 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 <berrange@redhat.com>
> ---
>  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);
> +
>  #endif
> 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 *data)
>          case QEVENT_WAKEUP:
>              event_name = "WAKEUP";
>              break;
> +        case QEVENT_BALLOON_CHANGE:
> +            event_name = "BALLOON_CHANGE";
> +            break;
>          default:
>              abort();
>              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 {
>      QEVENT_DEVICE_TRAY_MOVED,
>      QEVENT_SUSPEND,
>      QEVENT_WAKEUP,
> +    QEVENT_BALLOON_CHANGE,
>      QEVENT_MAX,
>  } MonitorEvent;
>