Add rmb() to tsc_read_##x to enforce serialization of rdtsc captures.

Intel Architecture Manual specifies that rdtsc instruction is not serialized,
so without this change, TSC synchronization test would periodically fail,
resulting in use of HPET timecounter instead of TSC-low.  This caused
severe performance degradation (40-50%) when running high IO/s workloads due to
HPET MMIO reads and GEOM stat collection.

Tests on Xeon E5-2600 (Sandy Bridge) 8C systems were seeing TSC synchronization
fail approximately 20% of the time.

Sponsored by: Intel
Reviewed by: kib
MFC after: 3 days

sys/x86/x86/tsc.c

@@ -328,6 +328,7 @@ init_TSC(void)
 
 #ifdef SMP
 
+/* rmb is required here because rdtsc is not a serializing instruction. */
 #define	TSC_READ(x)			\
 static void				\
 tsc_read_##x(void *arg)			\
@@ -335,6 +336,7 @@ tsc_read_##x(void *arg)			\
 	uint32_t *tsc = arg;		\
 	u_int cpu = PCPU_GET(cpuid);	\
 					\
+	rmb();				\
 	tsc[cpu * 3 + x] = rdtsc32();	\
 }
 TSC_READ(0)