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>

diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index 8f394eeb936eb124fea96eff9d749e32fcb7b277..9ad074c87e724f7b249c12196a1f8463e11bb33b 100644 (file)
--- a/arch/x86/kernel/tsc_sync.c
+++ b/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;
 }
 
 /*