x86, mm: Add pointer about Xen mmu requirement for alloc_low_pages

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	279b706b x86,xen: introduce x86_init.mapping.pagetable_reserve

-v2: updated to commets from hpa to include commit name.
Signed-off-by: NYinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1353123563-3103-29-git-send-email-yinghai@kernel.orgSigned-off-by: NH. Peter Anvin <hpa@linux.intel.com>

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;