diff --git a/tools/perf/Documentation/perf-kvm.txt b/tools/perf/Documentation/perf-kvm.txt
index ac84db2..6a06cef 100644
--- a/tools/perf/Documentation/perf-kvm.txt
+++ b/tools/perf/Documentation/perf-kvm.txt
@@ -109,7 +109,9 @@
 
 -m::
 --mmap-pages=::
-    Number of mmap data pages. Must be a power of two.
+    Number of mmap data pages (must be a power of two) or size
+    specification with appended unit character - B/K/M/G. The
+    size is rounded up to have nearest pages power of two value.
 
 -a::
 --all-cpus::