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Commit 09ec5442 authored by Thomas Gleixner's avatar Thomas Gleixner Committed by John Stultz
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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: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarJohn Stultz <john.stultz@linaro.org>
parent 3a978377

arch/x86/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -109,6 +109,7 @@ config X86 
	select CLOCKSOURCE_WATCHDOG

	select GENERIC_CLOCKEVENTS

	select ARCH_CLOCKSOURCE_DATA

	select CLOCKSOURCE_VALIDATE_LAST_CYCLE

	select GENERIC_CLOCKEVENTS_BROADCAST if X86_64 || (X86_32 && X86_LOCAL_APIC)

	select GENERIC_TIME_VSYSCALL

	select GENERIC_STRNCPY_FROM_USER

arch/x86/kernel/tsc.c

+9 −12
Original line number Diff line number Diff line
@@ -951,7 +951,7 @@ core_initcall(cpufreq_tsc); 
static struct clocksource clocksource_tsc;



/*

 * We compare the TSC to the cycle_last value in the clocksource

 * We used to 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
@@ -961,26 +961,23 @@ static struct clocksource clocksource_tsc; 
 * due to the unsigned delta calculation of the time keeping core

 * code, which is necessary to support wrapping clocksources like pm

 * timer.

 *

 * This sanity check is now done in the core timekeeping code.

 * checking the result of read_tsc() - cycle_last for being negative.

 * That works because CLOCKSOURCE_MASK(64) does not mask out any bit.

 */

static cycle_t read_tsc(struct clocksource *cs)

{

	cycle_t ret = (cycle_t)get_cycles();



	return ret >= clocksource_tsc.cycle_last ?

		ret : clocksource_tsc.cycle_last;

}



static void resume_tsc(struct clocksource *cs)

{

	if (!boot_cpu_has(X86_FEATURE_NONSTOP_TSC_S3))

		clocksource_tsc.cycle_last = 0;

	return (cycle_t)get_cycles();

}



/*

 * .mask MUST be CLOCKSOURCE_MASK(64). See comment above read_tsc()

 */

static struct clocksource clocksource_tsc = {

	.name                   = "tsc",

	.rating                 = 300,

	.read                   = read_tsc,

	.resume			= resume_tsc,

	.mask                   = CLOCKSOURCE_MASK(64),

	.flags                  = CLOCK_SOURCE_IS_CONTINUOUS |

				  CLOCK_SOURCE_MUST_VERIFY,

kernel/time/Kconfig

+5 −0
Original line number Diff line number Diff line
@@ -12,6 +12,11 @@ config CLOCKSOURCE_WATCHDOG 
config ARCH_CLOCKSOURCE_DATA

	bool



# Clocksources require validation of the clocksource against the last

# cycle update - x86/TSC misfeature

config CLOCKSOURCE_VALIDATE_LAST_CYCLE

	bool



# Timekeeping vsyscall support

config GENERIC_TIME_VSYSCALL

	bool

kernel/time/timekeeping_internal.h

+9 −0
Original line number Diff line number Diff line
@@ -12,9 +12,18 @@ extern void tk_debug_account_sleep_time(struct timespec64 *t); 
#define tk_debug_account_sleep_time(x)

#endif



#ifdef CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE

static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask)

{

	cycle_t ret = (now - last) & mask;



	return (s64) ret > 0 ? ret : 0;

}

#else

static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask)

{

	return (now - last) & mask;

}

#endif



#endif /* _TIMEKEEPING_INTERNAL_H */