[tip:x86/timers] x86/tsc: Detect random warps

[tip-bot for Thomas Gleixner]

Commit-ID:  1115b98b49f321c037b27b81f04ebf3147452ad6
Gitweb:     http://git.kernel.org/tip/1115b98b49f321c037b27b81f04ebf3147452ad6
Author:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Sat, 19 Nov 2016 13:47:35 +0000
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Tue, 29 Nov 2016 17:29:46 +0100

x86/tsc: Detect random warps

If time warps can be observed then they should only ever be observed on one
CPU. If they are observed on both CPUs then the system is completely hosed.

Add a check for this condition and notify if it happens.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Yinghai Lu <yinghai@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Link: http://lkml.kernel.org/r/20161119134017.574838461@xxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 arch/x86/kernel/tsc_sync.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index 78083bf..40f8edd 100644
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -37,6 +37,7 @@ static arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;
 static cycles_t last_tsc;
 static cycles_t max_warp;
 static int nr_warps;
+static int random_warps;
 
 /*
  * TSC-warp measurement loop running on both CPUs.  This is not called
@@ -45,7 +46,7 @@ static int nr_warps;
 static void check_tsc_warp(unsigned int timeout)
 {
 	cycles_t start, now, prev, end;
-	int i;
+	int i, cur_warps = 0;
 
 	start = rdtsc_ordered();
 	/*
@@ -85,7 +86,14 @@ static void check_tsc_warp(unsigned int timeout)
 		if (unlikely(prev > now)) {
 			arch_spin_lock(&sync_lock);
 			max_warp = max(max_warp, prev - now);
+			/*
+			 * Check whether this bounces back and forth. Only
+			 * one CPU should observe time going backwards.
+			 */
+			if (cur_warps != nr_warps)
+				random_warps++;
 			nr_warps++;
+			cur_warps = nr_warps;
 			arch_spin_unlock(&sync_lock);
 		}
 	}
@@ -160,6 +168,8 @@ void check_tsc_sync_source(int cpu)
 			smp_processor_id(), cpu);
 		pr_warning("Measured %Ld cycles TSC warp between CPUs, "
 			   "turning off TSC clock.\n", max_warp);
+		if (random_warps)
+			pr_warning("TSC warped randomly between CPUs\n");
 		mark_tsc_unstable("check_tsc_sync_source failed");
 	} else {
 		pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
@@ -170,6 +180,7 @@ void check_tsc_sync_source(int cpu)
 	 * Reset it - just in case we boot another CPU later:
 	 */
 	atomic_set(&start_count, 0);
+	random_warps = 0;
 	nr_warps = 0;
 	max_warp = 0;
 	last_tsc = 0;