virtio-balloon: return empty data when no stats are available

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:
https://www.redhat.com/archives/libvir-list/2014-May/msg00460.html

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 <jtomko@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

diff --git a/docs/virtio-balloon-stats.txt b/docs/virtio-balloon-stats.txt
index f74612f468dd4a11eb88fb4bfafeb5391d492a2a..edff5f22a8b79aa22a74d8e2978fc2b13b77e59f 100644 (file)
--- 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 bf2b588b24bbc4cd2c296314110d175a39437ebc..22cd52edeecc95b814053c16c9aebd00404cb3f1 100644 (file)
--- 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);