linux-kernel - [PATCH 3/9] x86/kernel/alternative.c: Add comment for applying alternatives order

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Message-Id: <1305671358-14478-4-git-send-email-fenghua.yu@intel.com>
Date:	Tue, 17 May 2011 15:29:12 -0700
From:	"Fenghua Yu" <fenghua.yu@...el.com>
To:	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"H Peter Anvin" <hpa@...or.com>,
	"Asit K Mallick" <asit.k.mallick@...el.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Avi Kivity" <avi@...hat.com>,
	"Arjan van de Ven" <arjan@...radead.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Andi Kleen" <andi@...stfloor.org>
Cc:	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"Fenghua Yu" <fenghua.yu@...el.com>
Subject: [PATCH 3/9] x86/kernel/alternative.c: Add comment for applying alternatives order

From: Fenghua Yu <fenghua.yu@...el.com>

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.

Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
---
 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 c0501ea..a81f2d5 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -266,6 +266,15 @@ 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);
-- 
1.7.2

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