[PATCH] x86: Add clflush before monitor for Intel 7400 series - v2

From: Pallipadi, Venkatesh
Date: Fri Feb 06 2009 - 19:48:25 EST




For Intel 7400 series CPUs, the recommendation is to use a clflush on the
monitored address just before monitor and mwait pair [1]. This clflush makes
sure that there are no false wakeups from mwait when the monitored address
was recently written to.

[1] "MONITOR/MWAIT Recommendations for Intel Xeon Processor 7400 series"
section in specification update document of 7400 series
http://download.intel.com/design/xeon/specupdt/32033601.pdf

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@xxxxxxxxx>

---

Earlier patch:
http://marc.info/?l=linux-kernel&m=122341330606025&w=2

Current changes:
Minor modification based on Peter's comment

arch/x86/include/asm/cpufeature.h | 1 +
arch/x86/kernel/cpu/intel.c | 3 +++
arch/x86/kernel/process.c | 6 ++++++
3 files changed, 10 insertions(+)

Index: linux-2.6/arch/x86/kernel/cpu/intel.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/cpu/intel.c 2009-02-06 15:58:25.000000000 -0800
+++ linux-2.6/arch/x86/kernel/cpu/intel.c 2009-02-06 16:11:41.000000000 -0800
@@ -291,6 +291,9 @@ static void __cpuinit init_intel(struct
ds_init_intel(c);
}

+ if (c->x86 == 6 && c->x86_model == 29 && cpu_has_clflush)
+ set_cpu_cap(c, X86_FEATURE_CLFLUSH_MONITOR);
+
#ifdef CONFIG_X86_64
if (c->x86 == 15)
c->x86_cache_alignment = c->x86_clflush_size * 2;
Index: linux-2.6/arch/x86/kernel/process.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/process.c 2009-02-03 12:58:38.000000000 -0800
+++ linux-2.6/arch/x86/kernel/process.c 2009-02-06 16:11:41.000000000 -0800
@@ -180,6 +180,9 @@ void mwait_idle_with_hints(unsigned long

trace_power_start(&it, POWER_CSTATE, (ax>>4)+1);
if (!need_resched()) {
+ if (cpu_has(&current_cpu_data, X86_FEATURE_CLFLUSH_MONITOR))
+ clflush((void *)&current_thread_info()->flags);
+
__monitor((void *)&current_thread_info()->flags, 0, 0);
smp_mb();
if (!need_resched())
@@ -194,6 +197,9 @@ static void mwait_idle(void)
struct power_trace it;
if (!need_resched()) {
trace_power_start(&it, POWER_CSTATE, 1);
+ if (cpu_has(&current_cpu_data, X86_FEATURE_CLFLUSH_MONITOR))
+ clflush((void *)&current_thread_info()->flags);
+
__monitor((void *)&current_thread_info()->flags, 0, 0);
smp_mb();
if (!need_resched())
Index: linux-2.6/arch/x86/include/asm/cpufeature.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/cpufeature.h 2009-02-06 16:11:12.000000000 -0800
+++ linux-2.6/arch/x86/include/asm/cpufeature.h 2009-02-06 16:40:44.000000000 -0800
@@ -93,6 +93,7 @@
#define X86_FEATURE_XTOPOLOGY (3*32+22) /* cpu topology enum extensions */
#define X86_FEATURE_TSC_RELIABLE (3*32+23) /* TSC is known to be reliable */
#define X86_FEATURE_NONSTOP_TSC (3*32+24) /* TSC does not stop in C states */
+#define X86_FEATURE_CLFLUSH_MONITOR (3*32+25) /* "" clflush reqd with monitor */

/* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
#define X86_FEATURE_XMM3 (4*32+ 0) /* "pni" SSE-3 */
--
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