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>

1 files changed, 5 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