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clocksource: Move cycle_last validation to core code
The only user of the cycle_last validation is the x86 TSC. In order to provide NMI safe accessor functions for clock monotonic and monotonic_raw we need to do that in the core. We can't do the TSC specific if (now < cycle_last) now = cycle_last; for the other wrapping around clocksources, but TSC has CLOCKSOURCE_MASK(64) which actually does not mask out anything so if now is less than cycle_last the subtraction will give a negative result. So we can check for that in clocksource_delta() and return 0 for that case. Implement and enable it for x86 Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: John Stultz <john.stultz@linaro.org>
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@ -109,6 +109,7 @@ config X86
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select CLOCKSOURCE_WATCHDOG
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select GENERIC_CLOCKEVENTS
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select ARCH_CLOCKSOURCE_DATA
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select CLOCKSOURCE_VALIDATE_LAST_CYCLE
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select GENERIC_CLOCKEVENTS_BROADCAST if X86_64 || (X86_32 && X86_LOCAL_APIC)
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select GENERIC_TIME_VSYSCALL
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select GENERIC_STRNCPY_FROM_USER
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@ -951,7 +951,7 @@ core_initcall(cpufreq_tsc);
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static struct clocksource clocksource_tsc;
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/*
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* We compare the TSC to the cycle_last value in the clocksource
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* We used to compare the TSC to the cycle_last value in the clocksource
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* structure to avoid a nasty time-warp. This can be observed in a
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* very small window right after one CPU updated cycle_last under
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* xtime/vsyscall_gtod lock and the other CPU reads a TSC value which
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@ -961,26 +961,23 @@ static struct clocksource clocksource_tsc;
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* due to the unsigned delta calculation of the time keeping core
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* code, which is necessary to support wrapping clocksources like pm
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* timer.
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*
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* This sanity check is now done in the core timekeeping code.
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* checking the result of read_tsc() - cycle_last for being negative.
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* That works because CLOCKSOURCE_MASK(64) does not mask out any bit.
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*/
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static cycle_t read_tsc(struct clocksource *cs)
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{
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cycle_t ret = (cycle_t)get_cycles();
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return ret >= clocksource_tsc.cycle_last ?
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ret : clocksource_tsc.cycle_last;
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}
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static void resume_tsc(struct clocksource *cs)
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{
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if (!boot_cpu_has(X86_FEATURE_NONSTOP_TSC_S3))
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clocksource_tsc.cycle_last = 0;
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return (cycle_t)get_cycles();
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}
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/*
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* .mask MUST be CLOCKSOURCE_MASK(64). See comment above read_tsc()
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*/
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static struct clocksource clocksource_tsc = {
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.name = "tsc",
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.rating = 300,
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.read = read_tsc,
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.resume = resume_tsc,
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.mask = CLOCKSOURCE_MASK(64),
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.flags = CLOCK_SOURCE_IS_CONTINUOUS |
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CLOCK_SOURCE_MUST_VERIFY,
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@ -12,6 +12,11 @@ config CLOCKSOURCE_WATCHDOG
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config ARCH_CLOCKSOURCE_DATA
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bool
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# Clocksources require validation of the clocksource against the last
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# cycle update - x86/TSC misfeature
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config CLOCKSOURCE_VALIDATE_LAST_CYCLE
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bool
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# Timekeeping vsyscall support
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config GENERIC_TIME_VSYSCALL
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bool
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@ -12,9 +12,18 @@ extern void tk_debug_account_sleep_time(struct timespec64 *t);
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#define tk_debug_account_sleep_time(x)
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#endif
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#ifdef CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE
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static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask)
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{
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cycle_t ret = (now - last) & mask;
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return (s64) ret > 0 ? ret : 0;
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}
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#else
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static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask)
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{
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return (now - last) & mask;
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}
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#endif
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#endif /* _TIMEKEEPING_INTERNAL_H */
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