perf report: Allow user to specify path to kallsyms file

This is useful for analyzing a perf data file on a different system than
the one data was collected on and still include symbols from loaded
kernel modules in the output.

Commiter note: Updated the man page accordingly.

LKML-Reference: <1291775986-16475-1-git-send-email-daahern@cisco.com>
Signed-off-by: David Ahern <daahern@cisco.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index a348906..f40c076 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -1830,8 +1830,8 @@
 	const char *kallsyms_filename = NULL;
 	char *kallsyms_allocated_filename = NULL;
 	/*
-	 * Step 1: if the user specified a vmlinux filename, use it and only
-	 * it, reporting errors to the user if it cannot be used.
+	 * Step 1: if the user specified a kallsyms or vmlinux filename, use
+	 * it and only it, reporting errors to the user if it cannot be used.
 	 *
 	 * For instance, try to analyse an ARM perf.data file _without_ a
 	 * build-id, or if the user specifies the wrong path to the right
@@ -1844,6 +1844,11 @@
 	 * validation in dso__load_vmlinux and will bail out if they don't
 	 * match.
 	 */
+	if (symbol_conf.kallsyms_name != NULL) {
+		kallsyms_filename = symbol_conf.kallsyms_name;
+		goto do_kallsyms;
+	}
+
 	if (symbol_conf.vmlinux_name != NULL) {
 		err = dso__load_vmlinux(self, map,
 					symbol_conf.vmlinux_name, filter);