[PATCH] kvm: update document of gfns

[Lai Jiangshan <laijs@cn.fujitsu.com>]

When don't use ->gfns when role.direct=1, update document for it.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Requested-by: Avi Kivity <avi@redhat.com>
---
diff --git a/Documentation/kvm/mmu.txt b/Documentation/kvm/mmu.txt
index da04671..aec19ae 100644
--- a/Documentation/kvm/mmu.txt
+++ b/Documentation/kvm/mmu.txt
@@ -178,7 +178,9 @@ Shadow pages contain the following information:
     guest pages as leaves.
   gfns:
     An array of 512 guest frame numbers, one for each present pte.  Used to
-    perform a reverse map from a pte to a gfn.
+    perform a reverse map from a pte to a gfn. When role.direct is set, any
+    element of this array can be calculated from the gfn field when used, in
+    this case, the array of gfns is not allocated. See role.direct and gfn.
   slot_bitmap:
     A bitmap containing one bit per memory slot.  If the page contains a pte
     mapping a page from memory slot n, then bit n of slot_bitmap will be set