x86/asm/tsc: Use rdtsc_ordered() in read_tsc() instead of get_cycles()

There are two logical changes here.  First, this removes a check
for cpu_has_tsc.  That check is unnecessary, as we don't
register the TSC as a clocksource on systems that have no TSC.

Second, it adds a barrier, thus preventing observable
non-monotonicity.

I suspect that the missing barrier was never a problem in
practice because system calls themselves were heavy enough
barriers to prevent user code from observing time warps due to
speculation. (Without the corresponding barrier in the vDSO,
however, non-monotonicity is easy to detect.)

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kvm ML <kvm@vger.kernel.org>
Link: http://lkml.kernel.org/r/c6ff621a053127a65b70f175443578db7a0711be.1434501121.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/tsc.c

@@ -961,7 +961,7 @@ static struct clocksource clocksource_tsc;
  */
 static cycle_t read_tsc(struct clocksource *cs)
 {
-	return (cycle_t)get_cycles();
+	return (cycle_t)rdtsc_ordered();
 }
 
 /*