diff options
Diffstat (limited to 'arch/x86/kernel/tsc_64.c')
| -rw-r--r-- | arch/x86/kernel/tsc_64.c | 23 |
1 files changed, 20 insertions, 3 deletions
diff --git a/arch/x86/kernel/tsc_64.c b/arch/x86/kernel/tsc_64.c index 947554ddabb..01fc9f0c39e 100644 --- a/arch/x86/kernel/tsc_64.c +++ b/arch/x86/kernel/tsc_64.c @@ -11,6 +11,7 @@ #include <asm/hpet.h> #include <asm/timex.h> #include <asm/timer.h> +#include <asm/vgtod.h> static int notsc __initdata = 0; @@ -290,18 +291,34 @@ int __init notsc_setup(char *s) __setup("notsc", notsc_setup); +static struct clocksource clocksource_tsc; -/* clock source code: */ +/* + * We compare the TSC to the cycle_last value in the clocksource + * structure to avoid a nasty time-warp. This can be observed in a + * very small window right after one CPU updated cycle_last under + * xtime/vsyscall_gtod 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 = (cycle_t)get_cycles(); - return ret; + + return ret >= clocksource_tsc.cycle_last ? + ret : clocksource_tsc.cycle_last; } static cycle_t __vsyscall_fn vread_tsc(void) { cycle_t ret = (cycle_t)vget_cycles(); - return ret; + + return ret >= __vsyscall_gtod_data.clock.cycle_last ? + ret : __vsyscall_gtod_data.clock.cycle_last; } static struct clocksource clocksource_tsc = { |