linux-kernel - [PATCH 1/2] x86: Enable fast strings on Intel if BIOS hasn't already

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Message-Id: <add0a25293cd8bea549e6aea4f38ccc279a64556.1312386029.git.luto@mit.edu>
Date:	Wed,  3 Aug 2011 11:47:16 -0400
From:	Andy Lutomirski <luto@....EDU>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Fenghua Yu <fenghua.yu@...el.com>, Andy Lutomirski <luto@....edu>
Subject: [PATCH 1/2] x86: Enable fast strings on Intel if BIOS hasn't already

I have two Sandy Bridge systems and neither one enables fast strings
in BIOS.  I can't find anything in the manual that says that only
BIOS should enable or disable the feature, so try to enable it at
bootup.

Signed-off-by: Andy Lutomirski <luto@....edu>
---
 arch/x86/kernel/cpu/intel.c |   17 +++++++++++++++--
 1 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index ed6086e..6b20fef 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -30,6 +30,7 @@
 static void __cpuinit early_init_intel(struct cpuinfo_x86 *c)
 {
 	u64 misc_enable;
+	bool allow_fast_string = true;
 
 	/* Unmask CPUID levels if masked: */
 	if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
@@ -118,10 +119,11 @@ static void __cpuinit early_init_intel(struct cpuinfo_x86 *c)
 	 * (model 2) with the same problem.
 	 */
 	if (c->x86 == 15) {
-		rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
+		allow_fast_string = false;
+		printk(KERN_INFO "kmemcheck: Disabling fast string operations\n");
 
+		rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
 		if (misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING) {
-			printk(KERN_INFO "kmemcheck: Disabling fast string operations\n");
 
 			misc_enable &= ~MSR_IA32_MISC_ENABLE_FAST_STRING;
 			wrmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
@@ -135,6 +137,17 @@ static void __cpuinit early_init_intel(struct cpuinfo_x86 *c)
 	 */
 	if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
 		rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
+
+		if (allow_fast_string &&
+		    !(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
+			misc_enable |= MSR_IA32_MISC_ENABLE_FAST_STRING;
+			wrmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
+			printk(KERN_INFO "Enabled fast string operations\n");
+
+			/* Re-read to make sure it stuck. */
+			rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
+		}
+
 		if (!(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
 			printk(KERN_INFO "Disabled fast string operations\n");
 			setup_clear_cpu_cap(X86_FEATURE_REP_GOOD);
-- 
1.7.6

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