[PATCH] x86_64: prevent auto select of mwait_idle for AMD CPUs

[Andreas Herrmann]

This fix is needed for AMD family 10h CPUs.

It prevents auto select of mwait_idle for AMD CPUs.
MWAIT does not enter C-states on family 10h and more
power saving is reached by entering C1 with
default_idle.

The patch also adds an idle=mwait command line option
to select mwait_idle for benchmarking.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@xxxxxxx>
---
 arch/x86_64/kernel/process.c    |    9 +++++++--
 arch/x86_64/kernel/setup.c      |    4 ++++
 include/asm-x86_64/cpufeature.h |    1 +
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index d8d5ccc..18fe9de 100644
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -271,7 +271,8 @@ static void mwait_idle(void)
 void __cpuinit select_idle_routine(const struct cpuinfo_x86 *c)
 {
 	static int printed;
-	if (cpu_has(c, X86_FEATURE_MWAIT)) {
+	if (cpu_has(c, X86_FEATURE_MWAIT) &&
+	    !cpu_has(c, X86_FEATURE_MWAIT_NO_CSTATE)) {
 		/*
 		 * Skip, if setup has overridden idle.
 		 * One CPU supports mwait => All CPUs supports mwait
@@ -291,7 +292,11 @@ static int __init idle_setup (char *str)
 	if (!strncmp(str, "poll", 4)) {
 		printk("using polling idle threads.\n");
 		pm_idle = poll_idle;
-	}
+	} else if (!strncmp(str, "mwait", 5))
+		if (boot_cpu_has(X86_FEATURE_MWAIT)) {
+			printk(KERN_INFO "using mwait in idle threads.\n");
+			pm_idle = mwait_idle;
+		}
 
 	boot_option_idle_override = 1;
 	return 1;
diff --git a/arch/x86_64/kernel/setup.c b/arch/x86_64/kernel/setup.c
index 3d98b69..b707025 100644
--- a/arch/x86_64/kernel/setup.c
+++ b/arch/x86_64/kernel/setup.c
@@ -612,6 +612,10 @@ static void __cpuinit init_amd(struct cpuinfo_x86 *c)
 
 	/* RDTSC can be speculated around */
 	clear_bit(X86_FEATURE_SYNC_RDTSC, &c->x86_capability);
+
+	/* prevent auto select of mwait_idle */
+	if (cpu_has(c, X86_FEATURE_MWAIT))
+		set_bit(X86_FEATURE_MWAIT_NO_CSTATE, c->x86_capability);
 }
 
 static void __cpuinit detect_ht(struct cpuinfo_x86 *c)
diff --git a/include/asm-x86_64/cpufeature.h b/include/asm-x86_64/cpufeature.h
index 0b3c686..9331d1a 100644
--- a/include/asm-x86_64/cpufeature.h
+++ b/include/asm-x86_64/cpufeature.h
@@ -70,6 +70,7 @@
 #define X86_FEATURE_ARCH_PERFMON (3*32+9) /* Intel Architectural PerfMon */
 #define X86_FEATURE_PEBS	(3*32+10) /* Precise-Event Based Sampling */
 #define X86_FEATURE_BTS		(3*32+11) /* Branch Trace Store */
+#define X86_FEATURE_MWAIT_NO_CSTATE (3*32+12) /* mwait does not enter c-state */
 
 /* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
 #define X86_FEATURE_XMM3	(4*32+ 0) /* Streaming SIMD Extensions-3 */
-- 
1.5.0.6




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