x86, hotplug: In the MWAIT case of play_dead, CLFLUSH the cache line

When we're using MWAIT for play_dead, explicitly CLFLUSH the cache
line before executing MONITOR.  This is a potential workaround for the
Xeon 7400 erratum AAI65 after having a spurious wakeup and returning
around the loop.  "Potential" here because it is not certain that that
erratum could actually trigger; however, the CLFLUSH should be
harmless.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Acked-by: Venkatesh Pallipadi <venki@google.com>
Cc: Asit Mallick <asit.k.mallick@intel.com>
Cc: Arjan van de Ven <arjan@linux.kernel.org>
Cc: Len Brown <lenb@kernel.org>

1 files changed, 19 insertions, 1 deletions

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 55c80ffb8719..fdccfe9dc63d 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1394,9 +1394,12 @@ static inline void mwait_play_dead(void)
         unsigned int highest_cstate = 0;
         unsigned int highest_subcstate = 0;
         int i;
+        void *mwait_ptr;
 
         if (!cpu_has(&current_cpu_data, X86_FEATURE_MWAIT))
                 return;
+        if (!cpu_has(&current_cpu_data, X86_FEATURE_CLFLSH))
+                return;
         if (current_cpu_data.cpuid_level < CPUID_MWAIT_LEAF)
                 return;
 
@@ -1422,10 +1425,25 @@ static inline void mwait_play_dead(void)
                         (highest_subcstate - 1);
         }
 
+        /*
+         * This should be a memory location in a cache line which is
+         * unlikely to be touched by other processors.  The actual
+         * content is immaterial as it is not actually modified in any way.
+         */
+        mwait_ptr = &current_thread_info()->flags;
+
         wbinvd();
 
         while (1) {
-                __monitor(&current_thread_info()->flags, 0, 0);
+                /*
+                 * The CLFLUSH is a workaround for erratum AAI65 for
+                 * the Xeon 7400 series.  It's not clear it is actually
+                 * 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.
+                 */
+                clflush(mwait_ptr);
+                __monitor(mwait_ptr, 0, 0);
                 mb();
                 __mwait(eax, 0);
         }