x86: read_tsc sync

make native_read_tsc() always non-speculative.
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/rtc.c

@@ -200,7 +200,10 @@ unsigned long long __vsyscall_fn native_read_tsc(void)
 {
 	DECLARE_ARGS(val, low, high);
 
+	rdtsc_barrier();
 	asm volatile("rdtsc" : EAX_EDX_RET(val, low, high));
+	rdtsc_barrier();
+
 	return EAX_EDX_VAL(val, low, high);
 }
 EXPORT_SYMBOL_GPL(native_read_tsc);