perf tools: Fix pmu events parsing rule

Currently all the event parsing fails end up
in the event_pmu rule, and display misleading
help like:

  $ perf stat -e inst kill
  event syntax error: 'inst'
                       \___ Cannot find PMU `inst'. Missing kernel support?
  ...

The reason is that the event_pmu is too strong
and match also single string. Changing it to
force the '/' separators to be part of the rule,
and getting the proper error now:

  $ perf stat -e inst kill
  event syntax error: 'inst'
                       \___ parser error
  Run 'perf list' for a list of valid events
  ...

Suggested-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180605121416.31645-1-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/parse-events.y b/tools/perf/util/parse-events.y
index e37608a..155d257 100644
--- a/tools/perf/util/parse-events.y
+++ b/tools/perf/util/parse-events.y
@@ -73,6 +73,7 @@
 %type <num> value_sym
 %type <head> event_config
 %type <head> opt_event_config
+%type <head> opt_pmu_config
 %type <term> event_term
 %type <head> event_pmu
 %type <head> event_legacy_symbol
@@ -224,7 +225,7 @@
 	   event_bpf_file
 
 event_pmu:
-PE_NAME opt_event_config
+PE_NAME opt_pmu_config
 {
 	struct list_head *list, *orig_terms, *terms;
 
@@ -496,6 +497,17 @@
 	$$ = NULL;
 }
 
+opt_pmu_config:
+'/' event_config '/'
+{
+	$$ = $2;
+}
+|
+'/' '/'
+{
+	$$ = NULL;
+}
+
 start_terms: event_config
 {
 	struct parse_events_state *parse_state = _parse_state;