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[Qemu-devel] [PATCH v2] virtio-balloon: return empty data when no stats

From: Ján Tomko
Subject: [Qemu-devel] [PATCH v2] virtio-balloon: return empty data when no stats are available
Date: Wed, 21 May 2014 11:03:47 +0200

If the guest hasn't updated the stats yet, instead of returning
an error, return '-1' for the stats and '0' as 'last-update'.

This lets applications ignore this without parsing the error message.

Related libvirt patch and discussion:

Tested against current upstream libvirt - stat reporting works and
it no longer logs errors when the stats are queried on domain startup.
(Note: libvirt doesn't use the last-update field for anything yet)

Signed-off-by: Ján Tomko <address@hidden>
v1: https://lists.gnu.org/archive/html/qemu-devel/2014-05/msg03643.html
  Reviewed-by: Eric Blake <address@hidden>
  added documentation changes
  mentioned testing in the commit message

 docs/virtio-balloon-stats.txt | 5 +++--
 hw/virtio/virtio-balloon.c    | 7 ++-----
 2 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/docs/virtio-balloon-stats.txt b/docs/virtio-balloon-stats.txt
index f74612f..edff5f2 100644
--- a/docs/virtio-balloon-stats.txt
+++ b/docs/virtio-balloon-stats.txt
@@ -35,7 +35,8 @@ which will return a dictionary containing:
   o A key named last-update, which contains the last stats update
     timestamp in seconds. Since this timestamp is generated by the host,
-    a buggy guest can't influence its value
+    a buggy guest can't influence its value. The value is 0 if the guest
+    has not updated the stats (yet).
 It's also important to note the following:
@@ -49,7 +50,7 @@ It's also important to note the following:
  - Polling can be enabled even if the guest doesn't have stats support
    or the balloon driver wasn't loaded in the guest. If this is the case
-   and stats are queried, an error will be returned
+   and stats are queried, last-update will be 0.
  - The polling timer is only re-armed when the guest responds to the
    statistics request. This means that if a (buggy) guest doesn't ever
diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index bf2b588..22cd52e 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -112,11 +112,6 @@ static void balloon_stats_get_all(Object *obj, struct 
Visitor *v,
     VirtIOBalloon *s = opaque;
     int i;
-    if (!s->stats_last_update) {
-        error_setg(errp, "guest hasn't updated any stats yet");
-        return;
-    }
     visit_start_struct(v, NULL, "guest-stats", name, 0, &err);
     if (err) {
         goto out;
@@ -378,6 +373,8 @@ static void virtio_balloon_device_realize(DeviceState *dev, 
Error **errp)
     s->dvq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
     s->svq = virtio_add_queue(vdev, 128, virtio_balloon_receive_stats);
+    reset_stats(s);
     register_savevm(dev, "virtio-balloon", -1, 1,
                     virtio_balloon_save, virtio_balloon_load, s);

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