timekeeping: Prepare timekeeping_cycles_to_ns() for overflow safety

Open code clocksource_delta() in timekeeping_cycles_to_ns() so that
overflow safety can be added efficiently.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240325064023.2997-17-adrian.hunter@intel.com

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 749387f22f0f14717370676747ba42a277e72df3..d17484082e2c95bbe05c9335e512180717159b1f 100644 (file)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -367,7 +367,17 @@ static void tk_setup_internals(struct timekeeper *tk, struct clocksource *clock)
 static inline u64 timekeeping_cycles_to_ns(const struct tk_read_base *tkr, u64 cycles)
 {
        /* Calculate the delta since the last update_wall_time() */
-       u64 delta = clocksource_delta(cycles, tkr->cycle_last, tkr->mask);
+       u64 mask = tkr->mask, delta = (cycles - tkr->cycle_last) & mask;
+
+       if (IS_ENABLED(CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE)) {
+               /*
+                * Handle clocksource inconsistency between CPUs to prevent
+                * time from going backwards by checking for the MSB of the
+                * mask being set in the delta.
+                */
+               if (unlikely(delta & ~(mask >> 1)))
+                       return tkr->xtime_nsec >> tkr->shift;
+       }
 
        return ((delta * tkr->mult) + tkr->xtime_nsec) >> tkr->shift;
 }