diff --git a/[refs] b/[refs]
index 645acee4e475..e79682a295d0 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 22c8ca2ac256bb681be791858b35502b5d37e73b
+refs/heads/master: ddd3509df8f8d4f1cf4784f559d702ce00dc8846
diff --git a/trunk/arch/x86/mm/init.c b/trunk/arch/x86/mm/init.c
index 02cea14c6d0c..cb4f8ba70ecc 100644
--- a/trunk/arch/x86/mm/init.c
+++ b/trunk/arch/x86/mm/init.c
@@ -25,6 +25,15 @@ unsigned long __meminitdata pgt_buf_top;
 
 static unsigned long min_pfn_mapped;
 
+/*
+ * Pages returned are already directly mapped.
+ *
+ * Changing that is likely to break Xen, see commit:
+ *
+ *    279b706 x86,xen: introduce x86_init.mapping.pagetable_reserve
+ *
+ * for detailed information.
+ */
 __ref void *alloc_low_pages(unsigned int num)
 {
 	unsigned long pfn;