[PATCH] x86/smpboot: check if CLFLUSH is actually necessary

[Scotty Bauer]

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 <sba...@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 <sba...@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