x86, alternative, doc: Add comment for applying alternatives order

Some string operation functions may be patched twice, e.g. on enhanced REP MOVSB
/STOSB processors, memcpy is patched first by fast string alternative function,
then it is patched by enhanced REP MOVSB/STOSB alternative function.

Add comment for applying alternatives order to warn people who may change the
applying alternatives order for any reason.

[ Documentation-only patch ]

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1305671358-14478-4-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 4a23467..f4fe15d 100644 (file)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -210,6 +210,15 @@ void __init_or_module apply_alternatives(struct alt_instr *start,
        u8 insnbuf[MAX_PATCH_LEN];
 
        DPRINTK("%s: alt table %p -> %p\n", __func__, start, end);
+       /*
+        * The scan order should be from start to end. A later scanned
+        * alternative code can overwrite a previous scanned alternative code.
+        * Some kernel functions (e.g. memcpy, memset, etc) use this order to
+        * patch code.
+        *
+        * So be careful if you want to change the scan order to any other
+        * order.
+        */
        for (a = start; a < end; a++) {
                u8 *instr = a->instr;
                BUG_ON(a->replacementlen > a->instrlen);