1 files changed, 18 insertions, 5 deletions

diff --git a/arch/x86/kernel/tsc_32.c b/arch/x86/kernel/tsc_32.c
index 3d7e6e9fa6c2..e4790728b224 100644
--- a/arch/x86/kernel/tsc_32.c
+++ b/arch/x86/kernel/tsc_32.c
@@ -221,9 +221,9 @@ EXPORT_SYMBOL(recalibrate_cpu_khz);
  * if the CPU frequency is scaled, TSC-based delays will need a different
  * loops_per_jiffy value to function properly.
  */
-static unsigned int ref_freq = 0;
-static unsigned long loops_per_jiffy_ref = 0;
-static unsigned long cpu_khz_ref = 0;
+static unsigned int ref_freq;
+static unsigned long loops_per_jiffy_ref;
+static unsigned long cpu_khz_ref;
 
 static int
 time_cpufreq_notifier(struct notifier_block *nb, unsigned long val, void *data)
@@ -283,15 +283,28 @@ core_initcall(cpufreq_tsc);
 
 /* clock source code */
 
-static unsigned long current_tsc_khz = 0;
+static unsigned long current_tsc_khz;
+static struct clocksource clocksource_tsc;
 
+/*
+ * We compare the TSC to the cycle_last value in the clocksource
+ * structure to avoid a nasty time-warp issue. This can be observed in
+ * a very small window right after one CPU updated cycle_last under
+ * xtime lock and the other CPU reads a TSC value which is smaller
+ * than the cycle_last reference value due to a TSC which is slighty
+ * behind. This delta is nowhere else observable, but in that case it
+ * results in a forward time jump in the range of hours due to the
+ * unsigned delta calculation of the time keeping core code, which is
+ * necessary to support wrapping clocksources like pm timer.
+ */
 static cycle_t read_tsc(void)
 {
         cycle_t ret;
 
         rdtscll(ret);
 
-        return ret;
+        return ret >= clocksource_tsc.cycle_last ?
+                ret : clocksource_tsc.cycle_last;
 }
 
 static struct clocksource clocksource_tsc = {