[tip:perf/core] x86, alternative, doc: Add comment for applying alternatives order

From: tip-bot for Fenghua Yu
Date: Wed May 18 2011 - 16:41:09 EST

Commit-ID: 509731336313b3799cf03071d72c64fa6383895e
Gitweb: http://git.kernel.org/tip/509731336313b3799cf03071d72c64fa6383895e
Author: Fenghua Yu <fenghua.yu@xxxxxxxxx>
AuthorDate: Tue, 17 May 2011 15:29:12 -0700
Committer: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
CommitDate: Tue, 17 May 2011 15:40:25 -0700

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@xxxxxxxxx>
Link: http://lkml.kernel.org/r/1305671358-14478-4-git-send-email-fenghua.yu@xxxxxxxxx
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
arch/x86/kernel/alternative.c | 9 +++++++++
1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 4a23467..f4fe15d 100644
--- 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);
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