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Subject[PATCH] x86/smpboot: check if CLFLUSH is actually necessary
mwait_play_dead previously issued a CLFLUSH to work around a bug on some xeon processors. We can now determine if the CPU is a buggy CPU. This patch checks if if we're on a buggy CPU which allows non-buggy cpu's to eliminate the CLFLUSH.





From 3da1be5c998a8d51f98fdba09b3cb477526c5ff3 Mon Sep 17 00:00:00 2001
From: Scott Bauer <sbauer@eng.utah.edu>
Date: Fri, 30 Jan 2015 14:10:37 -0700
Subject: [PATCH] Add check to determine if CLFLUSH is actually necessary
before monitor/wait

Signed-off-by: Scott Bauer <sbauer@eng.utah.edu>
---
arch/x86/kernel/smpboot.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 6d7022c..552ff48 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1384,6 +1384,7 @@ static inline void mwait_play_dead(void)
unsigned int highest_subcstate = 0;
void *mwait_ptr;
int i;
+ int cpu;

if (!this_cpu_has(X86_FEATURE_MWAIT))
return;
@@ -1420,6 +1421,7 @@ static inline void mwait_play_dead(void)
* content is immaterial as it is not actually modified in any way.
*/
mwait_ptr = &current_thread_info()->flags;
+ cpu = smp_processor_id();

wbinvd();

@@ -1430,10 +1432,15 @@ static inline void mwait_play_dead(void)
* needed, but it should be harmless in either case.
* The WBINVD is insufficient due to the spurious-wakeup
* case where we return around the loop.
+ *
+ * Check if the CLFLUSH is actually necessary before calling
*/
- mb();
- clflush(mwait_ptr);
- mb();
+ if (cpu_has_bug(&cpu_data(cpu), X86_BUG_CLFLUSH_MONITOR)) {
+ mb();
+ clflush(mwait_ptr);
+ mb();
+ }
+
__monitor(mwait_ptr, 0, 0);
mb();
__mwait(eax, 0);
--
1.9.1
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