Patchwork virtio: Add memory statistics reporting to the balloon driver (V6)

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Submitter Adam Litke
Date Jan. 11, 2010, 7:09 p.m.
Message ID <1263236975.2765.6.camel@aglitke>
Download mbox | patch
Permalink /patch/42653/
State New
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Comments

Adam Litke - Jan. 11, 2010, 7:09 p.m.
After some good discussion, V6 of this patch integrates well with the new QMP
support.  When the monitor is in QMP mode, the query-balloon command triggers a
stats refresh request to the guest.  This request is asynchronous.  If the
guest does not respond then nothing further happens.  When stats are updated, a
BALLOON monitor event is raised and the data element will contain the memory
statistics.

For the user monitor, a timer has been added to prevent monitor hangs with
unresponsive guests.  When the timer fires, the most recently collected stats
are returned along with an additional entry 'age' which indicates the number of
host_clock milliseconds that have passed since the stats were collected.

This method for dealing with asynchronous commands may prove useful for other
existing or future commands.  In that case, we may want to consider
incorporating this into the actual monitor API.

Changes since V5:
 - Asynchronous query-balloon mode for QMP
 - Add timeout to prevent hanging the user monitor in synchronous mode

Changes since V4:
 - Virtio spec updated: http://ozlabs.org/~rusty/virtio-spec/virtio-spec-0.8.2.pdf
 - Guest-side Linux implementation applied by Rusty
 - Start using the QObject infrastructure
 - All endian conversions done in the host
 - Report stats that reference a quantity of memory in bytes

Changes since V3:
 - Increase stat field size to 64 bits
 - Report all sizes in kb (not pages)
 - Drop anon_pages stat

Changes since V2:
 - Use a virtqueue for communication instead of the device config space

Changes since V1:
 - In the monitor, print all stats on one line with less abbreviated names
 - Coding style changes

When using ballooning to manage overcommitted memory on a host, a system for
guests to communicate their memory usage to the host can provide information
that will minimize the impact of ballooning on the guests.  The current method
employs a daemon running in each guest that communicates memory statistics to a
host daemon at a specified time interval.  The host daemon aggregates this
information and inflates and/or deflates balloons according to the level of
host memory pressure.  This approach is effective but overly complex since a
daemon must be installed inside each guest and coordinated to communicate with
the host.  A simpler approach is to collect memory statistics in the virtio
balloon driver and communicate them directly to the hypervisor.

Signed-off-by: Adam Litke <agl@us.ibm.com>
To: Anthony Liguori <aliguori@us.ibm.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Jamie Lokier <jamie@shareable.org>
Cc: qemu-devel@nongnu.org
Luiz Capitulino - Jan. 13, 2010, 6:04 p.m.
On Mon, 11 Jan 2010 13:09:35 -0600
Adam Litke <agl@us.ibm.com> wrote:

> After some good discussion, V6 of this patch integrates well with the new QMP
> support.  When the monitor is in QMP mode, the query-balloon command triggers a
> stats refresh request to the guest.  This request is asynchronous.  If the
> guest does not respond then nothing further happens.  When stats are updated, a
> BALLOON monitor event is raised and the data element will contain the memory
> statistics.
> 
> For the user monitor, a timer has been added to prevent monitor hangs with
> unresponsive guests.  When the timer fires, the most recently collected stats
> are returned along with an additional entry 'age' which indicates the number of
> host_clock milliseconds that have passed since the stats were collected.
> 
> This method for dealing with asynchronous commands may prove useful for other
> existing or future commands.  In that case, we may want to consider
> incorporating this into the actual monitor API.

 Yeah.

 Sorry for the delay in reviewing.

 I've tried to apply this patch to play with it, but turns out it conflicts
with recent changes in hw/virtio-balloon.

 Some comments on the QMP side of the patch follows.

> diff --git a/balloon.h b/balloon.h
> index 60b4a5d..7e29028 100644
> --- a/balloon.h
> +++ b/balloon.h
> @@ -16,12 +16,22 @@
>  
>  #include "cpu-defs.h"
>  
> -typedef ram_addr_t (QEMUBalloonEvent)(void *opaque, ram_addr_t target);
> +/* Timeout for synchronous stats requests (in seconds) */
> +#define QEMU_BALLOON_SYNC_TIMEOUT 5
> +
> +typedef enum {
> +    QEMU_BALLOON_MODE_NONE = 0,  /* No stats request is active */
> +    QEMU_BALLOON_MODE_SYNC = 1,  /* Synchronous stats request */
> +    QEMU_BALLOON_MODE_ASYNC = 2, /* Asynchronous stats request */
> +} balloon_mode_t;
> +
> +typedef void (QEMUBalloonEvent)(void *opaque, ram_addr_t target,
> +                                balloon_mode_t mode);
>  
>  void qemu_add_balloon_handler(QEMUBalloonEvent *func, void *opaque);
>  
> -void qemu_balloon(ram_addr_t target);
> +int qemu_balloon(ram_addr_t target);
>  
> -ram_addr_t qemu_balloon_status(void);
> +int qemu_balloon_status(balloon_mode_t mode);
>  
>  #endif
> diff --git a/hw/virtio-balloon.c b/hw/virtio-balloon.c
> index cfd3b41..bf67f4d 100644
> --- a/hw/virtio-balloon.c
> +++ b/hw/virtio-balloon.c
> @@ -19,6 +19,10 @@
>  #include "balloon.h"
>  #include "virtio-balloon.h"
>  #include "kvm.h"
> +#include "monitor.h"
> +#include "qlist.h"
> +#include "qint.h"
> +#include "qstring.h"
>  
>  #if defined(__linux__)
>  #include <sys/mman.h>
> @@ -27,9 +31,15 @@
>  typedef struct VirtIOBalloon
>  {
>      VirtIODevice vdev;
> -    VirtQueue *ivq, *dvq;
> +    VirtQueue *ivq, *dvq, *svq;
>      uint32_t num_pages;
>      uint32_t actual;
> +    uint64_t stats[VIRTIO_BALLOON_S_NR];
> +    VirtQueueElement stats_vq_elem;
> +    size_t stats_vq_offset;
> +    balloon_mode_t stats_request_mode;
> +    QEMUTimer *stats_timer;
> +    uint64_t stats_updated;
>  } VirtIOBalloon;
>  
>  static VirtIOBalloon *to_virtio_balloon(VirtIODevice *vdev)
> @@ -46,6 +56,50 @@ static void balloon_page(void *addr, int deflate)
>  #endif
>  }
>  
> +/*
> + * reset_stats - Mark all items in the stats array as unset
> + *
> + * This function needs to be called at device intialization and before
> + * before updating to a set of newly-generated stats.  This will ensure that no
> + * stale values stick around in case the guest reports a subset of the supported
> + * statistics.
> + */
> +static inline void reset_stats(VirtIOBalloon *dev)
> +{
> +    int i;
> +    for (i = 0; i < VIRTIO_BALLOON_S_NR; dev->stats[i++] = -1);
> +    dev->stats_updated = qemu_get_clock(host_clock);
> +}
> +
> +static void stat_put(QDict *dict, const char *label, uint64_t val)
> +{
> +    if (val != -1)
> +        qdict_put(dict, label, qint_from_int(val));
> +}
> +
> +static QObject *get_stats_qobject(VirtIOBalloon *dev)
> +{
> +    QDict *dict = qdict_new();
> +    uint32_t actual = ram_size - (dev->actual << VIRTIO_BALLOON_PFN_SHIFT);
> +    uint64_t age;
> +
> +    stat_put(dict, "actual", actual);
> +    stat_put(dict, "mem_swapped_in", dev->stats[VIRTIO_BALLOON_S_SWAP_IN]);
> +    stat_put(dict, "mem_swapped_out", dev->stats[VIRTIO_BALLOON_S_SWAP_OUT]);
> +    stat_put(dict, "major_page_faults", dev->stats[VIRTIO_BALLOON_S_MAJFLT]);
> +    stat_put(dict, "minor_page_faults", dev->stats[VIRTIO_BALLOON_S_MINFLT]);
> +    stat_put(dict, "free_mem", dev->stats[VIRTIO_BALLOON_S_MEMFREE]);
> +    stat_put(dict, "total_mem", dev->stats[VIRTIO_BALLOON_S_MEMTOT]);
> +
> +    /* If age is over the timeout threshold, report it */
> +    age = (qemu_get_clock(host_clock) - dev->stats_updated) /
> +          (get_ticks_per_sec() / 1000);
> +    if (age >= QEMU_BALLOON_SYNC_TIMEOUT * 1000)
> +        stat_put(dict, "age", age);
> +
> +    return QOBJECT(dict);
> +}
> +
>  /* FIXME: once we do a virtio refactoring, this will get subsumed into common
>   * code */
>  static size_t memcpy_from_iovector(void *data, size_t offset, size_t size,
> @@ -104,6 +158,65 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
>      }
>  }
>  
> +/*
> + * complete_stats_request - Clean up and report statistics.
> + */
> +static void complete_stats_request(VirtIOBalloon *vb)
> +{
> +    QObject *stats = get_stats_qobject(vb);
> +
> +    if (vb->stats_request_mode == QEMU_BALLOON_MODE_SYNC) {
> +        qemu_del_timer(vb->stats_timer);
> +        monitor_print_balloon(cur_mon, stats);
> +        monitor_resume(cur_mon);
> +    } else if (vb->stats_request_mode == QEMU_BALLOON_MODE_ASYNC) {
> +        monitor_protocol_event(QEVENT_BALLOON, stats);
> +    }
> +
> +    vb->stats_request_mode = QEMU_BALLOON_MODE_NONE;
> +}

 In the previous thread Anthony raised some issues about the 'cur_mon'
usage that made me concerned, because some important code rely on
it (read async events).

 As far as I could check, 'cur_mon' is guaranteed to be the default
Monitor which is fine if you're aware of it when putting QEMU to run,
but I'm afraid that testing your patch with two Monitors (user and qmp)
is not going to work.

 Maybe not a big deal, but would be good to be aware of potential
issues.


> +
> +/*
> + * stats_request_timeout - Timer callback for sychronous request timeout
> + *
> + * In the case of a synchronous timeout, just report the old statistics.
> + */
> +static void stats_request_timeout(void *opaque)
> +{
> +    VirtIOBalloon *vb = (VirtIOBalloon *)opaque;
> +    complete_stats_request(vb);
> +}
> +
> +static void virtio_balloon_receive_stats(VirtIODevice *vdev, VirtQueue *vq)
> +{
> +    VirtIOBalloon *s = DO_UPCAST(VirtIOBalloon, vdev, vdev);
> +    VirtQueueElement *elem = &s->stats_vq_elem;
> +    VirtIOBalloonStat stat;
> +    size_t offset = 0;
> +
> +    if (!virtqueue_pop(vq, elem))
> +        return;

 Coding style.

> +
> +    /* Initialize the stats to get rid of any stale values.  This is only
> +     * needed to handle the case where a guest supports fewer stats than it
> +     * used to (ie. it has booted into an old kernel).
> +     */
> +    reset_stats(s);
> +
> +    while (memcpy_from_iovector(&stat, offset, sizeof(stat), elem->out_sg,
> +                                elem->out_num) == sizeof(stat)) {
> +        uint16_t tag = tswap16(stat.tag);
> +        uint64_t val = tswap64(stat.val);
> +
> +        offset += sizeof(stat);
> +        if (tag < VIRTIO_BALLOON_S_NR)
> +            s->stats[tag] = val;
> +    }
> +    s->stats_vq_offset = offset;
> +
> +    complete_stats_request(s);
> +}
> +
>  static void virtio_balloon_get_config(VirtIODevice *vdev, uint8_t *config_data)
>  {
>      VirtIOBalloon *dev = to_virtio_balloon(vdev);
> @@ -126,10 +239,48 @@ static void virtio_balloon_set_config(VirtIODevice *vdev,
>  
>  static uint32_t virtio_balloon_get_features(VirtIODevice *vdev)
>  {
> -    return 0;
> +    return 1 << VIRTIO_BALLOON_F_STATS_VQ;
> +}
> +
> +/*
> + * request_stats - Initiate a request for memory statistics
> + *
> + * If stats are not supported, just report 'actual', otherwise, two types of
> + * requests are supported:
> + *
> + * Synchronous requests are intended to be used with the user monitor.  The
> + * monitor is suspended until the stats are received or the timer expires.
> + * Either the newly-update stats or the current stats (in the case of a timeout)
> + * are printed to the monitor and the monitor is resumed.
> + *
> + * Asynchronous requests are intended for QMP.  A QMP event will be generated
> + * only if the stats are updated.
> + */
> +static void request_stats(VirtIOBalloon *vb, balloon_mode_t mode)
> +{
> +    vb->stats_request_mode = mode;
> +
> +    if (mode == QEMU_BALLOON_MODE_SYNC) {
> +        uint64_t later = qemu_get_clock(vm_clock) +
> +                             QEMU_BALLOON_SYNC_TIMEOUT * get_ticks_per_sec();
> +        monitor_suspend(cur_mon);
> +        qemu_mod_timer(vb->stats_timer, later);
> +    }
> +
> +    if (vb->vdev.features & (1 << VIRTIO_BALLOON_F_STATS_VQ)) {
> +        virtqueue_push(vb->svq, &vb->stats_vq_elem, vb->stats_vq_offset);
> +        virtio_notify(&vb->vdev, vb->svq);
> +    } else {
> +        /* Stats are not supported.  Clear out any stale values that might have
> +         * been set by a more featureful guest kernel.
> +         */
> +        reset_stats(vb);
> +        complete_stats_request(vb);
> +    }
>  }
>  
> -static ram_addr_t virtio_balloon_to_target(void *opaque, ram_addr_t target)
> +static void virtio_balloon_to_target(void *opaque, ram_addr_t target,
> +                                     balloon_mode_t mode)
>  {
>      VirtIOBalloon *dev = opaque;
>  
> @@ -139,9 +290,9 @@ static ram_addr_t virtio_balloon_to_target(void *opaque, ram_addr_t target)
>      if (target) {
>          dev->num_pages = (ram_size - target) >> VIRTIO_BALLOON_PFN_SHIFT;
>          virtio_notify_config(&dev->vdev);
> +    } else {
> +        request_stats(dev, mode);
>      }
> -
> -    return ram_size - (dev->actual << VIRTIO_BALLOON_PFN_SHIFT);
>  }
>  
>  static void virtio_balloon_save(QEMUFile *f, void *opaque)
> @@ -152,6 +303,11 @@ static void virtio_balloon_save(QEMUFile *f, void *opaque)
>  
>      qemu_put_be32(f, s->num_pages);
>      qemu_put_be32(f, s->actual);
> +    qemu_put_buffer(f, (uint8_t *)&s->stats_vq_elem, sizeof(VirtQueueElement));
> +    qemu_put_buffer(f, (uint8_t *)&s->stats_vq_offset, sizeof(size_t));
> +    qemu_put_be32(f, s->stats_request_mode);
> +    qemu_put_timer(f, s->stats_timer);
> +    qemu_put_be32(f, s->stats_updated);
>  }
>  
>  static int virtio_balloon_load(QEMUFile *f, void *opaque, int version_id)
> @@ -165,6 +321,11 @@ static int virtio_balloon_load(QEMUFile *f, void *opaque, int version_id)
>  
>      s->num_pages = qemu_get_be32(f);
>      s->actual = qemu_get_be32(f);
> +    qemu_get_buffer(f, (uint8_t *)&s->stats_vq_elem, sizeof(VirtQueueElement));
> +    qemu_get_buffer(f, (uint8_t *)&s->stats_vq_offset, sizeof(size_t));
> +    s->stats_request_mode = qemu_get_be32(f);
> +    qemu_get_timer(f, s->stats_timer);
> +    s->stats_updated = qemu_get_be32(f);
>  
>      return 0;
>  }
> @@ -183,7 +344,10 @@ VirtIODevice *virtio_balloon_init(DeviceState *dev)
>  
>      s->ivq = virtio_add_queue(&s->vdev, 128, virtio_balloon_handle_output);
>      s->dvq = virtio_add_queue(&s->vdev, 128, virtio_balloon_handle_output);
> +    s->svq = virtio_add_queue(&s->vdev, 128, virtio_balloon_receive_stats);
>  
> +    s->stats_timer = qemu_new_timer(vm_clock, stats_request_timeout, s);
> +    reset_stats(s);
>      qemu_add_balloon_handler(virtio_balloon_to_target, s);
>  
>      register_savevm("virtio-balloon", -1, 1, virtio_balloon_save, virtio_balloon_load, s);
> diff --git a/hw/virtio-balloon.h b/hw/virtio-balloon.h
> index 9a0d119..e20cf6b 100644
> --- a/hw/virtio-balloon.h
> +++ b/hw/virtio-balloon.h
> @@ -25,6 +25,7 @@
>  
>  /* The feature bitmap for virtio balloon */
>  #define VIRTIO_BALLOON_F_MUST_TELL_HOST 0 /* Tell before reclaiming pages */
> +#define VIRTIO_BALLOON_F_STATS_VQ 1       /* Memory stats virtqueue */
>  
>  /* Size of a PFN in the balloon interface. */
>  #define VIRTIO_BALLOON_PFN_SHIFT 12
> @@ -37,4 +38,18 @@ struct virtio_balloon_config
>      uint32_t actual;
>  };
>  
> +/* Memory Statistics */
> +#define VIRTIO_BALLOON_S_SWAP_IN  0   /* Amount of memory swapped in */
> +#define VIRTIO_BALLOON_S_SWAP_OUT 1   /* Amount of memory swapped out */
> +#define VIRTIO_BALLOON_S_MAJFLT   2   /* Number of major faults */
> +#define VIRTIO_BALLOON_S_MINFLT   3   /* Number of minor faults */
> +#define VIRTIO_BALLOON_S_MEMFREE  4   /* Total amount of free memory */
> +#define VIRTIO_BALLOON_S_MEMTOT   5   /* Total amount of memory */
> +#define VIRTIO_BALLOON_S_NR       6
> +
> +typedef struct VirtIOBalloonStat {
> +    uint16_t tag;
> +    uint64_t val;
> +} __attribute__((packed)) VirtIOBalloonStat;
> +
>  #endif
> diff --git a/monitor.c b/monitor.c
> index c0dc48e..3f3bbfb 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -358,6 +358,9 @@ void monitor_protocol_event(MonitorEvent event, QObject *data)
>          case QEVENT_STOP:
>              event_name = "STOP";
>              break;
> +        case QEVENT_BALLOON:
> +            event_name = "BALLOON";
> +            break;
>          default:
>              abort();
>              break;
> @@ -2058,43 +2061,27 @@ static void do_info_status(Monitor *mon, QObject **ret_data)
>                                      vm_running, singlestep);
>  }
>  
> -static ram_addr_t balloon_get_value(void)
> +static void print_balloon_stat(const char *key, QObject *obj, void *opaque)
>  {
> -    ram_addr_t actual;
> -
> -    if (kvm_enabled() && !kvm_has_sync_mmu()) {
> -        qemu_error_new(QERR_KVM_MISSING_CAP, "synchronous MMU", "balloon");
> -        return 0;
> -    }
> -
> -    actual = qemu_balloon_status();
> -    if (actual == 0) {
> -        qemu_error_new(QERR_DEVICE_NOT_ACTIVE, "balloon");
> -        return 0;
> -    }
> -
> -    return actual;
> -}
> +    Monitor *mon = opaque;
>  
> -/**
> - * do_balloon(): Request VM to change its memory allocation
> - */
> -static void do_balloon(Monitor *mon, const QDict *qdict, QObject **ret_data)
> -{
> -    if (balloon_get_value()) {
> -        /* ballooning is active */
> -        qemu_balloon(qdict_get_int(qdict, "value"));
> -    }
> +    if (strcmp(key, "actual"))
> +        monitor_printf(mon, ",%s=%" PRId64, key,
> +                            qint_get_int(qobject_to_qint(obj)));
>  }
>  
> -static void monitor_print_balloon(Monitor *mon, const QObject *data)
> +void monitor_print_balloon(Monitor *mon, const QObject *data)
>  {
>      QDict *qdict;
>  
>      qdict = qobject_to_qdict(data);
> +    if (!qdict_haskey(qdict, "actual"))
> +        return;
>  
> -    monitor_printf(mon, "balloon: actual=%" PRId64 "\n",
> -                        qdict_get_int(qdict, "balloon") >> 20);
> +    monitor_printf(mon, "balloon: actual=%" PRId64,
> +                        qdict_get_int(qdict, "actual") >> 20);
> +    qdict_iter(qdict, print_balloon_stat, mon);
> +    monitor_printf(mon, "\n");
>  }
>  
>  /**
> @@ -2102,21 +2089,59 @@ static void monitor_print_balloon(Monitor *mon, const QObject *data)
>   *
>   * Return a QDict with the following information:
>   *
> - * - "balloon": current balloon value in bytes
> + * - "actual": current balloon value in bytes
> + * The following fields may or may not be present:
> + * - "mem_swapped_in": Amount of memory swapped in (bytes)
> + * - "mem_swapped_out": Amount of memory swapped out (bytes)
> + * - "major_page_faults": Number of major faults
> + * - "minor_page_faults": Number of minor faults
> + * - "free_mem": Total amount of free and unused memory (bytes)
> + * - "total_mem": Total amount of available memory (bytes)
>   *
>   * Example:
>   *
> - * { "balloon": 1073741824 }
> + * { "actual": 1073741824, "mem_swapped_in": 0, "mem_swapped_out": 0,
> + *   "major_page_faults": 142, "minor_page_faults": 239245,
> + *   "free_mem": 1014185984, "total_mem": 1044668416 }
>   */
>  static void do_info_balloon(Monitor *mon, QObject **ret_data)
>  {
> -    ram_addr_t actual;
> +    int ret;
> +    balloon_mode_t mode;
>  
> -    actual = balloon_get_value();
> -    if (actual != 0) {
> -        *ret_data = qobject_from_jsonf("{ 'balloon': %" PRId64 "}",
> -                                       (int64_t) actual);
> +    if (kvm_enabled() && !kvm_has_sync_mmu()) {
> +        qemu_error_new(QERR_KVM_MISSING_CAP, "synchronous MMU", "balloon");
> +        return;
> +    }
> +
> +    if (monitor_ctrl_mode(mon))
> +        mode = QEMU_BALLOON_MODE_ASYNC;
> +    else
> +        
> +    mode = monitor_ctrl_mode(mon) ?
> +                            QEMU_BALLOON_MODE_ASYNC : QEMU_BALLOON_MODE_SYNC;

 I think what you want is:

} else {
 mode = QEMU_BALLOON_MODE_SYNC;
}

> +    ret = qemu_balloon_status(mode);
> +    if (!ret) {
> +        qemu_error_new(QERR_DEVICE_NOT_ACTIVE, "balloon");
> +        return;
> +    }
> +}
> +
> +/**
> + * do_balloon(): Request VM to change its memory allocation
> + */
> +static void do_balloon(Monitor *mon, const QDict *qdict, QObject **ret_data)
> +{
> +    int ret;
> +
> +    if (kvm_enabled() && !kvm_has_sync_mmu()) {
> +        qemu_error_new(QERR_KVM_MISSING_CAP, "synchronous MMU", "balloon");
> +        return;
>      }
> +
> +    ret = qemu_balloon(qdict_get_int(qdict, "value"));
> +    if (ret == 0)
> +        qemu_error_new(QERR_DEVICE_NOT_ACTIVE, "balloon");
>  }
>  
>  static qemu_acl *find_acl(Monitor *mon, const char *name)
> diff --git a/monitor.h b/monitor.h
> index 6ed117a..addb631 100644
> --- a/monitor.h
> +++ b/monitor.h
> @@ -20,6 +20,7 @@ typedef enum MonitorEvent {
>      QEVENT_RESET,
>      QEVENT_POWERDOWN,
>      QEVENT_STOP,
> +    QEVENT_BALLOON,
>      QEVENT_MAX,
>  } MonitorEvent;
>  
> @@ -32,6 +33,7 @@ void monitor_resume(Monitor *mon);
>  void monitor_read_bdrv_key_start(Monitor *mon, BlockDriverState *bs,
>                                   BlockDriverCompletionFunc *completion_cb,
>                                   void *opaque);
> +void monitor_print_balloon(Monitor *mon, const QObject *data);
>  
>  int monitor_get_fd(Monitor *mon, const char *fdname);
>  
> diff --git a/vl.c b/vl.c
> index 8be3648..44ef75d 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -362,17 +362,25 @@ void qemu_add_balloon_handler(QEMUBalloonEvent *func, void *opaque)
>      qemu_balloon_event_opaque = opaque;
>  }
>  
> -void qemu_balloon(ram_addr_t target)
> +int qemu_balloon(ram_addr_t target)
>  {
> -    if (qemu_balloon_event)
> -        qemu_balloon_event(qemu_balloon_event_opaque, target);
> +    if (qemu_balloon_event) {
> +        qemu_balloon_event(qemu_balloon_event_opaque, target,
> +                           QEMU_BALLOON_MODE_SYNC);
> +        return 1;
> +    } else {
> +        return 0;
> +    }
>  }

 This is used by do_balloon() right? Which is also used by QMP,
shouldn't it also handle async vs. sync?

>  
> -ram_addr_t qemu_balloon_status(void)
> +int qemu_balloon_status(balloon_mode_t mode)
>  {
> -    if (qemu_balloon_event)
> -        return qemu_balloon_event(qemu_balloon_event_opaque, 0);
> -    return 0;
> +    if (qemu_balloon_event) {
> +        qemu_balloon_event(qemu_balloon_event_opaque, 0, mode);
> +        return 1;
> +    } else {
> +        return 0;
> +    }
>  }
>  
>  /***********************************************************/
> 
>
Adam Litke - Jan. 13, 2010, 6:59 p.m.
On Wed, 2010-01-13 at 16:04 -0200, Luiz Capitulino wrote:
>  I've tried to apply this patch to play with it, but turns out it conflicts
> with recent changes in hw/virtio-balloon.

Ahh, I will continue my never-ending quest to stay current :)

>  Some comments on the QMP side of the patch follows.

Thanks for your review!

<snip>

> > +/*
> > + * complete_stats_request - Clean up and report statistics.
> > + */
> > +static void complete_stats_request(VirtIOBalloon *vb)
> > +{
> > +    QObject *stats = get_stats_qobject(vb);
> > +
> > +    if (vb->stats_request_mode == QEMU_BALLOON_MODE_SYNC) {
> > +        qemu_del_timer(vb->stats_timer);
> > +        monitor_print_balloon(cur_mon, stats);
> > +        monitor_resume(cur_mon);
> > +    } else if (vb->stats_request_mode == QEMU_BALLOON_MODE_ASYNC) {
> > +        monitor_protocol_event(QEVENT_BALLOON, stats);
> > +    }
> > +
> > +    vb->stats_request_mode = QEMU_BALLOON_MODE_NONE;
> > +}
> 
>  In the previous thread Anthony raised some issues about the 'cur_mon'
> usage that made me concerned, because some important code rely on
> it (read async events).
> 
>  As far as I could check, 'cur_mon' is guaranteed to be the default
> Monitor which is fine if you're aware of it when putting QEMU to run,
> but I'm afraid that testing your patch with two Monitors (user and qmp)
> is not going to work.
> 
>  Maybe not a big deal, but would be good to be aware of potential
> issues.

I talked to Anthony and will try to fix this up.  I just need to start
passing the monitor pointer around as well.

<snip>
      * 
> > +    if (monitor_ctrl_mode(mon))
> > +        mode = QEMU_BALLOON_MODE_ASYNC;
> > +    else
> > +        
> > +    mode = monitor_ctrl_mode(mon) ?
> > +                            QEMU_BALLOON_MODE_ASYNC : QEMU_BALLOON_MODE_SYNC;
> 
>  I think what you want is:
> 
> } else {
>  mode = QEMU_BALLOON_MODE_SYNC;
> }

Bah... Left over gunk. 

<snip>

> > -void qemu_balloon(ram_addr_t target)
> > +int qemu_balloon(ram_addr_t target)
> >  {
> > -    if (qemu_balloon_event)
> > -        qemu_balloon_event(qemu_balloon_event_opaque, target);
> > +    if (qemu_balloon_event) {
> > +        qemu_balloon_event(qemu_balloon_event_opaque, target,
> > +                           QEMU_BALLOON_MODE_SYNC);
> > +        return 1;
> > +    } else {
> > +        return 0;
> > +    }
> >  }
> 
>  This is used by do_balloon() right? Which is also used by QMP,
> shouldn't it also handle async vs. sync?

qemu_balloon always acts synchronously.  It does not wait on the guest
to do anything and it does not return data.
Luiz Capitulino - Jan. 13, 2010, 7:05 p.m.
On Wed, 13 Jan 2010 12:59:25 -0600
Adam Litke <agl@us.ibm.com> wrote:

> > > +/*
> > > + * complete_stats_request - Clean up and report statistics.
> > > + */
> > > +static void complete_stats_request(VirtIOBalloon *vb)
> > > +{
> > > +    QObject *stats = get_stats_qobject(vb);
> > > +
> > > +    if (vb->stats_request_mode == QEMU_BALLOON_MODE_SYNC) {
> > > +        qemu_del_timer(vb->stats_timer);
> > > +        monitor_print_balloon(cur_mon, stats);
> > > +        monitor_resume(cur_mon);
> > > +    } else if (vb->stats_request_mode == QEMU_BALLOON_MODE_ASYNC) {
> > > +        monitor_protocol_event(QEVENT_BALLOON, stats);
> > > +    }
> > > +
> > > +    vb->stats_request_mode = QEMU_BALLOON_MODE_NONE;
> > > +}
> > 
> >  In the previous thread Anthony raised some issues about the 'cur_mon'
> > usage that made me concerned, because some important code rely on
> > it (read async events).
> > 
> >  As far as I could check, 'cur_mon' is guaranteed to be the default
> > Monitor which is fine if you're aware of it when putting QEMU to run,
> > but I'm afraid that testing your patch with two Monitors (user and qmp)
> > is not going to work.
> > 
> >  Maybe not a big deal, but would be good to be aware of potential
> > issues.
> 
> I talked to Anthony and will try to fix this up.  I just need to start
> passing the monitor pointer around as well.

 After doing that you could help us improving the Monitor and check
the other cur_mon usages too :)

> > > -void qemu_balloon(ram_addr_t target)
> > > +int qemu_balloon(ram_addr_t target)
> > >  {
> > > -    if (qemu_balloon_event)
> > > -        qemu_balloon_event(qemu_balloon_event_opaque, target);
> > > +    if (qemu_balloon_event) {
> > > +        qemu_balloon_event(qemu_balloon_event_opaque, target,
> > > +                           QEMU_BALLOON_MODE_SYNC);
> > > +        return 1;
> > > +    } else {
> > > +        return 0;
> > > +    }
> > >  }
> > 
> >  This is used by do_balloon() right? Which is also used by QMP,
> > shouldn't it also handle async vs. sync?
> 
> qemu_balloon always acts synchronously.  It does not wait on the guest
> to do anything and it does not return data.

 Fine then.

Patch

diff --git a/balloon.h b/balloon.h
index 60b4a5d..7e29028 100644
--- a/balloon.h
+++ b/balloon.h
@@ -16,12 +16,22 @@ 
 
 #include "cpu-defs.h"
 
-typedef ram_addr_t (QEMUBalloonEvent)(void *opaque, ram_addr_t target);
+/* Timeout for synchronous stats requests (in seconds) */
+#define QEMU_BALLOON_SYNC_TIMEOUT 5
+
+typedef enum {
+    QEMU_BALLOON_MODE_NONE = 0,  /* No stats request is active */
+    QEMU_BALLOON_MODE_SYNC = 1,  /* Synchronous stats request */
+    QEMU_BALLOON_MODE_ASYNC = 2, /* Asynchronous stats request */
+} balloon_mode_t;
+
+typedef void (QEMUBalloonEvent)(void *opaque, ram_addr_t target,
+                                balloon_mode_t mode);
 
 void qemu_add_balloon_handler(QEMUBalloonEvent *func, void *opaque);
 
-void qemu_balloon(ram_addr_t target);
+int qemu_balloon(ram_addr_t target);
 
-ram_addr_t qemu_balloon_status(void);
+int qemu_balloon_status(balloon_mode_t mode);
 
 #endif
diff --git a/hw/virtio-balloon.c b/hw/virtio-balloon.c
index cfd3b41..bf67f4d 100644
--- a/hw/virtio-balloon.c
+++ b/hw/virtio-balloon.c
@@ -19,6 +19,10 @@ 
 #include "balloon.h"
 #include "virtio-balloon.h"
 #include "kvm.h"
+#include "monitor.h"
+#include "qlist.h"
+#include "qint.h"
+#include "qstring.h"
 
 #if defined(__linux__)
 #include <sys/mman.h>
@@ -27,9 +31,15 @@ 
 typedef struct VirtIOBalloon
 {
     VirtIODevice vdev;
-    VirtQueue *ivq, *dvq;
+    VirtQueue *ivq, *dvq, *svq;
     uint32_t num_pages;
     uint32_t actual;
+    uint64_t stats[VIRTIO_BALLOON_S_NR];
+    VirtQueueElement stats_vq_elem;
+    size_t stats_vq_offset;
+    balloon_mode_t stats_request_mode;
+    QEMUTimer *stats_timer;
+    uint64_t stats_updated;
 } VirtIOBalloon;
 
 static VirtIOBalloon *to_virtio_balloon(VirtIODevice *vdev)
@@ -46,6 +56,50 @@  static void balloon_page(void *addr, int deflate)
 #endif
 }
 
+/*
+ * reset_stats - Mark all items in the stats array as unset
+ *
+ * This function needs to be called at device intialization and before
+ * before updating to a set of newly-generated stats.  This will ensure that no
+ * stale values stick around in case the guest reports a subset of the supported
+ * statistics.
+ */
+static inline void reset_stats(VirtIOBalloon *dev)
+{
+    int i;
+    for (i = 0; i < VIRTIO_BALLOON_S_NR; dev->stats[i++] = -1);
+    dev->stats_updated = qemu_get_clock(host_clock);
+}
+
+static void stat_put(QDict *dict, const char *label, uint64_t val)
+{
+    if (val != -1)
+        qdict_put(dict, label, qint_from_int(val));
+}
+
+static QObject *get_stats_qobject(VirtIOBalloon *dev)
+{
+    QDict *dict = qdict_new();
+    uint32_t actual = ram_size - (dev->actual << VIRTIO_BALLOON_PFN_SHIFT);
+    uint64_t age;
+
+    stat_put(dict, "actual", actual);
+    stat_put(dict, "mem_swapped_in", dev->stats[VIRTIO_BALLOON_S_SWAP_IN]);
+    stat_put(dict, "mem_swapped_out", dev->stats[VIRTIO_BALLOON_S_SWAP_OUT]);
+    stat_put(dict, "major_page_faults", dev->stats[VIRTIO_BALLOON_S_MAJFLT]);
+    stat_put(dict, "minor_page_faults", dev->stats[VIRTIO_BALLOON_S_MINFLT]);
+    stat_put(dict, "free_mem", dev->stats[VIRTIO_BALLOON_S_MEMFREE]);
+    stat_put(dict, "total_mem", dev->stats[VIRTIO_BALLOON_S_MEMTOT]);
+
+    /* If age is over the timeout threshold, report it */
+    age = (qemu_get_clock(host_clock) - dev->stats_updated) /
+          (get_ticks_per_sec() / 1000);
+    if (age >= QEMU_BALLOON_SYNC_TIMEOUT * 1000)
+        stat_put(dict, "age", age);
+
+    return QOBJECT(dict);
+}
+
 /* FIXME: once we do a virtio refactoring, this will get subsumed into common
  * code */
 static size_t memcpy_from_iovector(void *data, size_t offset, size_t size,
@@ -104,6 +158,65 @@  static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
     }
 }
 
+/*
+ * complete_stats_request - Clean up and report statistics.
+ */
+static void complete_stats_request(VirtIOBalloon *vb)
+{
+    QObject *stats = get_stats_qobject(vb);
+
+    if (vb->stats_request_mode == QEMU_BALLOON_MODE_SYNC) {
+        qemu_del_timer(vb->stats_timer);
+        monitor_print_balloon(cur_mon, stats);
+        monitor_resume(cur_mon);
+    } else if (vb->stats_request_mode == QEMU_BALLOON_MODE_ASYNC) {
+        monitor_protocol_event(QEVENT_BALLOON, stats);
+    }
+
+    vb->stats_request_mode = QEMU_BALLOON_MODE_NONE;
+}
+
+/*
+ * stats_request_timeout - Timer callback for sychronous request timeout
+ *
+ * In the case of a synchronous timeout, just report the old statistics.
+ */
+static void stats_request_timeout(void *opaque)
+{
+    VirtIOBalloon *vb = (VirtIOBalloon *)opaque;
+    complete_stats_request(vb);
+}
+
+static void virtio_balloon_receive_stats(VirtIODevice *vdev, VirtQueue *vq)
+{
+    VirtIOBalloon *s = DO_UPCAST(VirtIOBalloon, vdev, vdev);
+    VirtQueueElement *elem = &s->stats_vq_elem;
+    VirtIOBalloonStat stat;
+    size_t offset = 0;
+
+    if (!virtqueue_pop(vq, elem))
+        return;
+
+    /* Initialize the stats to get rid of any stale values.  This is only
+     * needed to handle the case where a guest supports fewer stats than it
+     * used to (ie. it has booted into an old kernel).
+     */
+    reset_stats(s);
+
+    while (memcpy_from_iovector(&stat, offset, sizeof(stat), elem->out_sg,
+                                elem->out_num) == sizeof(stat)) {
+        uint16_t tag = tswap16(stat.tag);
+        uint64_t val = tswap64(stat.val);
+
+        offset += sizeof(stat);
+        if (tag < VIRTIO_BALLOON_S_NR)
+            s->stats[tag] = val;
+    }
+    s->stats_vq_offset = offset;
+
+    complete_stats_request(s);
+}
+
 static void virtio_balloon_get_config(VirtIODevice *vdev, uint8_t *config_data)
 {
     VirtIOBalloon *dev = to_virtio_balloon(vdev);
@@ -126,10 +239,48 @@  static void virtio_balloon_set_config(VirtIODevice *vdev,
 
 static uint32_t virtio_balloon_get_features(VirtIODevice *vdev)
 {
-    return 0;
+    return 1 << VIRTIO_BALLOON_F_STATS_VQ;
+}
+
+/*
+ * request_stats - Initiate a request for memory statistics
+ *
+ * If stats are not supported, just report 'actual', otherwise, two types of
+ * requests are supported:
+ *
+ * Synchronous requests are intended to be used with the user monitor.  The
+ * monitor is suspended until the stats are received or the timer expires.
+ * Either the newly-update stats or the current stats (in the case of a timeout)
+ * are printed to the monitor and the monitor is resumed.
+ *
+ * Asynchronous requests are intended for QMP.  A QMP event will be generated
+ * only if the stats are updated.
+ */
+static void request_stats(VirtIOBalloon *vb, balloon_mode_t mode)
+{
+    vb->stats_request_mode = mode;
+
+    if (mode == QEMU_BALLOON_MODE_SYNC) {
+        uint64_t later = qemu_get_clock(vm_clock) +
+                             QEMU_BALLOON_SYNC_TIMEOUT * get_ticks_per_sec();
+        monitor_suspend(cur_mon);
+        qemu_mod_timer(vb->stats_timer, later);
+    }
+
+    if (vb->vdev.features & (1 << VIRTIO_BALLOON_F_STATS_VQ)) {
+        virtqueue_push(vb->svq, &vb->stats_vq_elem, vb->stats_vq_offset);
+        virtio_notify(&vb->vdev, vb->svq);
+    } else {
+        /* Stats are not supported.  Clear out any stale values that might have
+         * been set by a more featureful guest kernel.
+         */
+        reset_stats(vb);
+        complete_stats_request(vb);
+    }
 }
 
-static ram_addr_t virtio_balloon_to_target(void *opaque, ram_addr_t target)
+static void virtio_balloon_to_target(void *opaque, ram_addr_t target,
+                                     balloon_mode_t mode)
 {
     VirtIOBalloon *dev = opaque;
 
@@ -139,9 +290,9 @@  static ram_addr_t virtio_balloon_to_target(void *opaque, ram_addr_t target)
     if (target) {
         dev->num_pages = (ram_size - target) >> VIRTIO_BALLOON_PFN_SHIFT;
         virtio_notify_config(&dev->vdev);
+    } else {
+        request_stats(dev, mode);
     }
-
-    return ram_size - (dev->actual << VIRTIO_BALLOON_PFN_SHIFT);
 }
 
 static void virtio_balloon_save(QEMUFile *f, void *opaque)
@@ -152,6 +303,11 @@  static void virtio_balloon_save(QEMUFile *f, void *opaque)
 
     qemu_put_be32(f, s->num_pages);
     qemu_put_be32(f, s->actual);
+    qemu_put_buffer(f, (uint8_t *)&s->stats_vq_elem, sizeof(VirtQueueElement));
+    qemu_put_buffer(f, (uint8_t *)&s->stats_vq_offset, sizeof(size_t));
+    qemu_put_be32(f, s->stats_request_mode);
+    qemu_put_timer(f, s->stats_timer);
+    qemu_put_be32(f, s->stats_updated);
 }
 
 static int virtio_balloon_load(QEMUFile *f, void *opaque, int version_id)
@@ -165,6 +321,11 @@  static int virtio_balloon_load(QEMUFile *f, void *opaque, int version_id)
 
     s->num_pages = qemu_get_be32(f);
     s->actual = qemu_get_be32(f);
+    qemu_get_buffer(f, (uint8_t *)&s->stats_vq_elem, sizeof(VirtQueueElement));
+    qemu_get_buffer(f, (uint8_t *)&s->stats_vq_offset, sizeof(size_t));
+    s->stats_request_mode = qemu_get_be32(f);
+    qemu_get_timer(f, s->stats_timer);
+    s->stats_updated = qemu_get_be32(f);
 
     return 0;
 }
@@ -183,7 +344,10 @@  VirtIODevice *virtio_balloon_init(DeviceState *dev)
 
     s->ivq = virtio_add_queue(&s->vdev, 128, virtio_balloon_handle_output);
     s->dvq = virtio_add_queue(&s->vdev, 128, virtio_balloon_handle_output);
+    s->svq = virtio_add_queue(&s->vdev, 128, virtio_balloon_receive_stats);
 
+    s->stats_timer = qemu_new_timer(vm_clock, stats_request_timeout, s);
+    reset_stats(s);
     qemu_add_balloon_handler(virtio_balloon_to_target, s);
 
     register_savevm("virtio-balloon", -1, 1, virtio_balloon_save, virtio_balloon_load, s);
diff --git a/hw/virtio-balloon.h b/hw/virtio-balloon.h
index 9a0d119..e20cf6b 100644
--- a/hw/virtio-balloon.h
+++ b/hw/virtio-balloon.h
@@ -25,6 +25,7 @@ 
 
 /* The feature bitmap for virtio balloon */
 #define VIRTIO_BALLOON_F_MUST_TELL_HOST 0 /* Tell before reclaiming pages */
+#define VIRTIO_BALLOON_F_STATS_VQ 1       /* Memory stats virtqueue */
 
 /* Size of a PFN in the balloon interface. */
 #define VIRTIO_BALLOON_PFN_SHIFT 12
@@ -37,4 +38,18 @@  struct virtio_balloon_config
     uint32_t actual;
 };
 
+/* Memory Statistics */
+#define VIRTIO_BALLOON_S_SWAP_IN  0   /* Amount of memory swapped in */
+#define VIRTIO_BALLOON_S_SWAP_OUT 1   /* Amount of memory swapped out */
+#define VIRTIO_BALLOON_S_MAJFLT   2   /* Number of major faults */
+#define VIRTIO_BALLOON_S_MINFLT   3   /* Number of minor faults */
+#define VIRTIO_BALLOON_S_MEMFREE  4   /* Total amount of free memory */
+#define VIRTIO_BALLOON_S_MEMTOT   5   /* Total amount of memory */
+#define VIRTIO_BALLOON_S_NR       6
+
+typedef struct VirtIOBalloonStat {
+    uint16_t tag;
+    uint64_t val;
+} __attribute__((packed)) VirtIOBalloonStat;
+
 #endif
diff --git a/monitor.c b/monitor.c
index c0dc48e..3f3bbfb 100644
--- a/monitor.c
+++ b/monitor.c
@@ -358,6 +358,9 @@  void monitor_protocol_event(MonitorEvent event, QObject *data)
         case QEVENT_STOP:
             event_name = "STOP";
             break;
+        case QEVENT_BALLOON:
+            event_name = "BALLOON";
+            break;
         default:
             abort();
             break;
@@ -2058,43 +2061,27 @@  static void do_info_status(Monitor *mon, QObject **ret_data)
                                     vm_running, singlestep);
 }
 
-static ram_addr_t balloon_get_value(void)
+static void print_balloon_stat(const char *key, QObject *obj, void *opaque)
 {
-    ram_addr_t actual;
-
-    if (kvm_enabled() && !kvm_has_sync_mmu()) {
-        qemu_error_new(QERR_KVM_MISSING_CAP, "synchronous MMU", "balloon");
-        return 0;
-    }
-
-    actual = qemu_balloon_status();
-    if (actual == 0) {
-        qemu_error_new(QERR_DEVICE_NOT_ACTIVE, "balloon");
-        return 0;
-    }
-
-    return actual;
-}
+    Monitor *mon = opaque;
 
-/**
- * do_balloon(): Request VM to change its memory allocation
- */
-static void do_balloon(Monitor *mon, const QDict *qdict, QObject **ret_data)
-{
-    if (balloon_get_value()) {
-        /* ballooning is active */
-        qemu_balloon(qdict_get_int(qdict, "value"));
-    }
+    if (strcmp(key, "actual"))
+        monitor_printf(mon, ",%s=%" PRId64, key,
+                            qint_get_int(qobject_to_qint(obj)));
 }
 
-static void monitor_print_balloon(Monitor *mon, const QObject *data)
+void monitor_print_balloon(Monitor *mon, const QObject *data)
 {
     QDict *qdict;
 
     qdict = qobject_to_qdict(data);
+    if (!qdict_haskey(qdict, "actual"))
+        return;
 
-    monitor_printf(mon, "balloon: actual=%" PRId64 "\n",
-                        qdict_get_int(qdict, "balloon") >> 20);
+    monitor_printf(mon, "balloon: actual=%" PRId64,
+                        qdict_get_int(qdict, "actual") >> 20);
+    qdict_iter(qdict, print_balloon_stat, mon);
+    monitor_printf(mon, "\n");
 }
 
 /**
@@ -2102,21 +2089,59 @@  static void monitor_print_balloon(Monitor *mon, const QObject *data)
  *
  * Return a QDict with the following information:
  *
- * - "balloon": current balloon value in bytes
+ * - "actual": current balloon value in bytes
+ * The following fields may or may not be present:
+ * - "mem_swapped_in": Amount of memory swapped in (bytes)
+ * - "mem_swapped_out": Amount of memory swapped out (bytes)
+ * - "major_page_faults": Number of major faults
+ * - "minor_page_faults": Number of minor faults
+ * - "free_mem": Total amount of free and unused memory (bytes)
+ * - "total_mem": Total amount of available memory (bytes)
  *
  * Example:
  *
- * { "balloon": 1073741824 }
+ * { "actual": 1073741824, "mem_swapped_in": 0, "mem_swapped_out": 0,
+ *   "major_page_faults": 142, "minor_page_faults": 239245,
+ *   "free_mem": 1014185984, "total_mem": 1044668416 }
  */
 static void do_info_balloon(Monitor *mon, QObject **ret_data)
 {
-    ram_addr_t actual;
+    int ret;
+    balloon_mode_t mode;
 
-    actual = balloon_get_value();
-    if (actual != 0) {
-        *ret_data = qobject_from_jsonf("{ 'balloon': %" PRId64 "}",
-                                       (int64_t) actual);
+    if (kvm_enabled() && !kvm_has_sync_mmu()) {
+        qemu_error_new(QERR_KVM_MISSING_CAP, "synchronous MMU", "balloon");
+        return;
+    }
+
+    if (monitor_ctrl_mode(mon))
+        mode = QEMU_BALLOON_MODE_ASYNC;
+    else
+        
+    mode = monitor_ctrl_mode(mon) ?
+                            QEMU_BALLOON_MODE_ASYNC : QEMU_BALLOON_MODE_SYNC;
+    ret = qemu_balloon_status(mode);
+    if (!ret) {
+        qemu_error_new(QERR_DEVICE_NOT_ACTIVE, "balloon");
+        return;
+    }
+}
+
+/**
+ * do_balloon(): Request VM to change its memory allocation
+ */
+static void do_balloon(Monitor *mon, const QDict *qdict, QObject **ret_data)
+{
+    int ret;
+
+    if (kvm_enabled() && !kvm_has_sync_mmu()) {
+        qemu_error_new(QERR_KVM_MISSING_CAP, "synchronous MMU", "balloon");
+        return;
     }
+
+    ret = qemu_balloon(qdict_get_int(qdict, "value"));
+    if (ret == 0)
+        qemu_error_new(QERR_DEVICE_NOT_ACTIVE, "balloon");
 }
 
 static qemu_acl *find_acl(Monitor *mon, const char *name)
diff --git a/monitor.h b/monitor.h
index 6ed117a..addb631 100644
--- a/monitor.h
+++ b/monitor.h
@@ -20,6 +20,7 @@  typedef enum MonitorEvent {
     QEVENT_RESET,
     QEVENT_POWERDOWN,
     QEVENT_STOP,
+    QEVENT_BALLOON,
     QEVENT_MAX,
 } MonitorEvent;
 
@@ -32,6 +33,7 @@  void monitor_resume(Monitor *mon);
 void monitor_read_bdrv_key_start(Monitor *mon, BlockDriverState *bs,
                                  BlockDriverCompletionFunc *completion_cb,
                                  void *opaque);
+void monitor_print_balloon(Monitor *mon, const QObject *data);
 
 int monitor_get_fd(Monitor *mon, const char *fdname);
 
diff --git a/vl.c b/vl.c
index 8be3648..44ef75d 100644
--- a/vl.c
+++ b/vl.c
@@ -362,17 +362,25 @@  void qemu_add_balloon_handler(QEMUBalloonEvent *func, void *opaque)
     qemu_balloon_event_opaque = opaque;
 }
 
-void qemu_balloon(ram_addr_t target)
+int qemu_balloon(ram_addr_t target)
 {
-    if (qemu_balloon_event)
-        qemu_balloon_event(qemu_balloon_event_opaque, target);
+    if (qemu_balloon_event) {
+        qemu_balloon_event(qemu_balloon_event_opaque, target,
+                           QEMU_BALLOON_MODE_SYNC);
+        return 1;
+    } else {
+        return 0;
+    }
 }
 
-ram_addr_t qemu_balloon_status(void)
+int qemu_balloon_status(balloon_mode_t mode)
 {
-    if (qemu_balloon_event)
-        return qemu_balloon_event(qemu_balloon_event_opaque, 0);
-    return 0;
+    if (qemu_balloon_event) {
+        qemu_balloon_event(qemu_balloon_event_opaque, 0, mode);
+        return 1;
+    } else {
+        return 0;
+    }
 }
 
 /***********************************************************/