x86: add clflush before monitor for Intel 7400 series

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@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index e68bb9e30864..6d12f7e37f8c 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -180,6 +180,9 @@ void mwait_idle_with_hints(unsigned long ax, unsigned long cx)
 
         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())