docs: Document QMP event rate limiting

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1444921716-9511-8-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

docs/qmp-events.txt

@@ -28,6 +28,8 @@ Example:
     "data": { "actual": 944766976 },
     "timestamp": { "seconds": 1267020223, "microseconds": 435656 } }
 
+Note: this event is rate-limited.
+
 BLOCK_IMAGE_CORRUPTED
 ---------------------
 
@@ -296,6 +298,8 @@ Example:
      "data": { "reference": "usr1", "sector-num": 345435, "sectors-count": 5 },
      "timestamp": { "seconds": 1344522075, "microseconds": 745528 } }
 
+Note: this event is rate-limited.
+
 QUORUM_REPORT_BAD
 -----------------
 
@@ -318,6 +322,8 @@ Example:
      "data": { "node-name": "1.raw", "sector-num": 345435, "sectors-count": 5 },
      "timestamp": { "seconds": 1344522075, "microseconds": 745528 } }
 
+Note: this event is rate-limited.
+
 RESET
 -----
 
@@ -358,6 +364,8 @@ Example:
     "data": { "offset": 78 },
     "timestamp": { "seconds": 1267020223, "microseconds": 435656 } }
 
+Note: this event is rate-limited.
+
 SHUTDOWN
 --------
 
@@ -632,6 +640,8 @@ Example:
     "data": { "id": "channel0", "open": true },
     "timestamp": { "seconds": 1401385907, "microseconds": 422329 } }
 
+Note: this event is rate-limited separately for each "id".
+
 WAKEUP
 ------
 
@@ -662,3 +672,5 @@ Example:
 
 Note: If action is "reset", "shutdown", or "pause" the WATCHDOG event is
 followed respectively by the RESET, SHUTDOWN, or STOP events.
+
+Note: this event is rate-limited.

docs/qmp-spec.txt

@@ -175,6 +175,11 @@ The format of asynchronous events is:
 For a listing of supported asynchronous events, please, refer to the
 qmp-events.txt file.
 
+Some events are rate-limited to at most one per second.  If additional
+"similar" events arrive within one second, all but the last one are
+dropped, and the last one is delayed.  "Similar" normally means same
+event type.  See qmp-events.txt for details.
+
 2.5 QGA Synchronization
 -----------------------