trace: document '-' syntax for disabling events

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

diff --git a/docs/tracing.txt b/docs/tracing.txt
index c54113336896a688b29f611f29ffddd84dd28571..79014093db7ccff651e7984ccef9bacfd5d62bb7 100644 (file)
--- a/docs/tracing.txt
+++ b/docs/tracing.txt
@@ -139,6 +139,10 @@ having a common prefix in a batch. For example, virtio-blk trace events could
 be enabled using:
   trace-event virtio_blk_* on
 
+If a line in the "-trace events=<file>" file begins with a '-', the trace event
+will be disabled instead of enabled.  This is useful when a wildcard was used
+to enable an entire family of events but one noisy event needs to be disabled.
+
 == Trace backends ==
 
 The "tracetool" script automates tedious trace event code generation and also