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>

2 files changed, 14 insertions, 0 deletions

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index feccfd888732..d626dc98e8df 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -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
diff --git a/kernel/time/timekeeping_internal.h b/kernel/time/timekeeping_internal.h
index 05dfa6b25dc4..4ea005a7f9da 100644
--- a/kernel/time/timekeeping_internal.h
+++ b/kernel/time/timekeeping_internal.h
@@ -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 */