[PATCH] x86_64: Don't try to synchronize the TSC over CPUs on Intel CPUs at boot.

They already do this in hardware and the Linux algorithm
actually adds errors.

Cc: mingo@elte.hu
Cc: rohit.seth@intel.com

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/arch/x86_64/kernel/smpboot.c b/arch/x86_64/kernel/smpboot.c
index e6af93b51dce..994728564d6b 100644
--- a/arch/x86_64/kernel/smpboot.c
+++ b/arch/x86_64/kernel/smpboot.c
@@ -335,7 +335,13 @@ static __cpuinit void sync_tsc(unsigned int master)
 
 static void __cpuinit tsc_sync_wait(void)
 {
-        if (notscsync || !cpu_has_tsc)
+        /*
+         * When the CPU has synchronized TSCs assume the BIOS
+         * or the hardware already synced.  Otherwise we could
+         * mess up a possible perfect synchronization with a
+         * not-quite-perfect algorithm.
+         */
+        if (notscsync || !cpu_has_tsc || !unsynchronized_tsc())
                 return;
         sync_tsc(0);
 }