diff options
Diffstat (limited to 'arch/x86/kernel/tsc_32.c')
| -rw-r--r-- | arch/x86/kernel/tsc_32.c | 15 |
1 files changed, 14 insertions, 1 deletions
diff --git a/arch/x86/kernel/tsc_32.c b/arch/x86/kernel/tsc_32.c index 06af8cf8251f..e4790728b224 100644 --- a/arch/x86/kernel/tsc_32.c +++ b/arch/x86/kernel/tsc_32.c @@ -284,14 +284,27 @@ core_initcall(cpufreq_tsc); /* clock source code */ 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 = { |