x86/tsc: Prepare warp test for TSC adjustment

To allow TSC compensation cross nodes its necessary to know in which
direction the TSC warp was observed. Return the maximum observed value on
the calling CPU so the caller can determine the direction later.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Link: http://lkml.kernel.org/r/20161119134017.970859287@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/tsc_sync.c

@@ -166,9 +166,9 @@ static int random_warps;
  * TSC-warp measurement loop running on both CPUs.  This is not called
  * if there is no TSC.
  */
-static void check_tsc_warp(unsigned int timeout)
+static cycles_t check_tsc_warp(unsigned int timeout)
 {
-	cycles_t start, now, prev, end;
+	cycles_t start, now, prev, end, cur_max_warp = 0;
 	int i, cur_warps = 0;
 
 	start = rdtsc_ordered();
@@ -209,6 +209,7 @@ static void check_tsc_warp(unsigned int timeout)
 		if (unlikely(prev > now)) {
 			arch_spin_lock(&sync_lock);
 			max_warp = max(max_warp, prev - now);
+			cur_max_warp = max_warp;
 			/*
 			 * Check whether this bounces back and forth. Only
 			 * one CPU should observe time going backwards.
@@ -223,6 +224,7 @@ static void check_tsc_warp(unsigned int timeout)
 	WARN(!(now-start),
 		"Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
 			now-start, end-start);
+	return cur_max_warp;
 }
 
 /*