/* clock source code */
static unsigned long current_tsc_khz = 0;
+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 = {