[tip:x86/mm2] x86, mm: Add pointer about Xen mmu requirement for alloc_low_pages

[tip-bot for Stefano Stabellini]

Commit-ID:  ddd3509df8f8d4f1cf4784f559d702ce00dc8846
Gitweb:     http://git.kernel.org/tip/ddd3509df8f8d4f1cf4784f559d702ce00dc8846
Author:     Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
AuthorDate: Fri, 16 Nov 2012 19:39:05 -0800
Committer:  H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
CommitDate: Sat, 17 Nov 2012 11:59:28 -0800

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

Add link for more information
	279b706 x86,xen: introduce x86_init.mapping.pagetable_reserve

-v2: updated to commets from hpa to include commit name.

Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/1353123563-3103-29-git-send-email-yinghai@xxxxxxxxxx
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
---
 arch/x86/mm/init.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index 02cea14..cb4f8ba 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;
--
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