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, 10 insertions, 12 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 7fa17b5ce668..d08e061c187a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -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
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 57e5ce126d5a..456c0e660c43 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -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,