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 f74612f..edff5f2 100644
--- a/docs/virtio-balloon-stats.txt
+++ b/docs/virtio-balloon-stats.txt
@@ -35,7 +35,8 @@
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 @@
- 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
