docs: document virtio-balloon stats

Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

docs/virtio-balloon-stats.txt

+virtio balloon memory statistics
+================================
+
+The virtio balloon driver supports guest memory statistics reporting. These
+statistics are available to QEMU users as QOM (QEMU Object Model) device
+properties via a polling mechanism.
+
+Before querying the available stats, clients first have to enable polling.
+This is done by writing a time interval value (in seconds) to the
+guest-stats-polling-interval property. This value can be:
+
+  > 0  enables polling in the specified interval. If polling is already
+       enabled, the polling time interval is changed to the new value
+
+  0    disables polling. Previous polled statistics are still valid and
+       can be queried.
+
+Once polling is enabled, the virtio-balloon device in QEMU will start
+polling the guest's balloon driver for new stats in the specified time
+interval.
+
+To retrieve those stats, clients have to query the guest-stats property,
+which will return a dictionary containing:
+
+  o A key named 'stats', containing all available stats. If the guest
+    doesn't support a particular stat, or if it couldn't be retrieved,
+    its value will be -1. Currently, the following stats are supported:
+
+      - stat-swap-in
+      - stat-swap-out
+      - stat-major-faults
+      - stat-minor-faults
+      - stat-free-memory
+      - stat-total-memory
+
+  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
+
+It's also important to note the following:
+
+ - Previously polled statistics remain available even if the polling is
+   later disabled
+
+ - As noted above, if a guest doesn't support a particular stat its value
+   will always be -1. However, it's also possible that a guest temporarily
+   couldn't update one or even all stats. If this happens, just wait for
+   the next update
+
+ - 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
+
+ - 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
+   respond to the request the timer will never be re-armed, which has
+   the same effect as disabling polling
+
+Here are a few examples. QEMU is started with '-balloon virtio', which
+generates '/machine/peripheral-anon/device[1]' as the QOM path for the
+balloon device.
+
+Enable polling with 2 seconds interval:
+
+{ "execute": "qom-set",
+             "arguments": { "path": "/machine/peripheral-anon/device[1]",
+			 "property": "guest-stats-polling-interval", "value": 2 } }
+
+{ "return": {} }
+
+Change polling to 10 seconds:
+
+{ "execute": "qom-set",
+             "arguments": { "path": "/machine/peripheral-anon/device[1]",
+			 "property": "guest-stats-polling-interval", "value": 10 } }
+
+{ "return": {} }
+
+Get stats:
+
+{ "execute": "qom-get",
+  "arguments": { "path": "/machine/peripheral-anon/device[1]",
+  "property": "guest-stats" } }
+{
+    "return": {
+        "stats": {
+            "stat-swap-out": 0,
+            "stat-free-memory": 844943360,
+            "stat-minor-faults": 219028,
+            "stat-major-faults": 235,
+            "stat-total-memory": 1044406272,
+            "stat-swap-in": 0
+        },
+        "last-update": 1358529861
+    }
+}
+
+Disable polling:
+
+{ "execute": "qom-set",
+             "arguments": { "path": "/machine/peripheral-anon/device[1]",
+			 "property": "stats-polling-interval", "value": 0 } }
+
+{ "return": {} }