linux-kernel - [PATCH 2/7] x86/jump-label: Do not bother updating nops if they are correct

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Date:	Thu, 08 Mar 2012 17:17:32 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>
Subject: [PATCH 2/7] x86/jump-label: Do not bother updating nops if they are correct

From: Steven Rostedt <srostedt@...hat.com>

On boot up, the jump label init function scans all the jump label locations
and converts them to the best nop for the machine. If the nop is already
the ideal nop, do not bother with changing it.

Cc: Jason Baron <jbaron@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
---
 arch/x86/kernel/jump_label.c |   20 +++++++++++++++++++-
 1 files changed, 19 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
index 2889b3d..11cc4da 100644
--- a/arch/x86/kernel/jump_label.c
+++ b/arch/x86/kernel/jump_label.c
@@ -53,7 +53,25 @@ void arch_jump_label_transform(struct jump_entry *entry,
 __init_or_module void arch_jump_label_transform_static(struct jump_entry *entry,
 				      enum jump_label_type type)
 {
-	__jump_label_transform(entry, type, text_poke_early);
+	static int once;
+	static int update;
+
+	/*
+	 * This function is called at boot up and when modules are
+	 * first loaded. Check if the default nop, the one that is
+	 * inserted at compile time, is the ideal nop. If it is, then
+	 * we do not need to update the nop, and we can leave it as is.
+	 * If it is not, then we need to update the nop to the ideal nop.
+	 */
+	if (!once) {
+		const unsigned char default_nop[] = { STATIC_KEY_INIT_NOP };
+		const unsigned char *ideal_nop = ideal_nops[NOP_ATOMIC5];
+		once++;
+		if (memcmp(ideal_nop, default_nop, 5) != 0)
+			update = 1;
+	}
+	if (update)
+		__jump_label_transform(entry, type, text_poke_early);
 }
 
 #endif
-- 
1.7.8.3



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