diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index 02cea14c6d0c14dbfff24fa328eb0d24eedbf908..cb4f8ba70eccba475e67e9de9fd0dbc75fef593c 100644
--- a/arch/x86/mm/init.c
+++ b/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;