i386: Add warning in Documentation that zero-page is not a stable ABI

Some people writing boot loaders seem to falsely belief the 32bit zero page is a
stable interface for out of tree code like the real mode boot protocol. Add a comment
clarifying that is not true.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/Documentation/i386/zero-page.txt b/Documentation/i386/zero-page.txt
index 75b3680c41eb835e751a242191f2439914166152..6c0817c45683fd07f6367c8234d419a4e824ee48 100644 (file)
--- a/Documentation/i386/zero-page.txt
+++ b/Documentation/i386/zero-page.txt
@@ -1,3 +1,13 @@
+---------------------------------------------------------------------------
+!!!!!!!!!!!!!!!WARNING!!!!!!!!
+The zero page is a kernel internal data structure, not a stable ABI.  It might change
+without warning and the kernel has no way to detect old version of it.
+If you're writing some external code like a boot loader you should only use
+the stable versioned real mode boot protocol described in boot.txt. Otherwise the kernel
+might break you at any time.
+!!!!!!!!!!!!!WARNING!!!!!!!!!!!
+----------------------------------------------------------------------------
+
 Summary of boot_params layout (kernel point of view)
      ( collected by Hans Lermen and Martin Mares )