timekeeping: Always check for negative motion

clocksource_delta() has two variants. One with a check for negative motion,
which is only selected by x86. This is a historic leftover as this function
was previously used in the time getter hot paths.

Since 135225a timekeeping_cycles_to_ns() has unconditional protection
against this as a by-product of the protection against 64bit math overflow.

clocksource_delta() is only used in the clocksource watchdog and in
timekeeping_advance(). The extra conditional there is not hurting anyone.

Remove the config option and unconditionally prevent negative motion of the
readout.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20241031120328.599430157@linutronix.de

arch/x86/Kconfig

@@ -145,7 +145,6 @@ config X86
 	select ARCH_HAS_PARANOID_L1D_FLUSH
 	select BUILDTIME_TABLE_SORT
 	select CLKEVT_I8253
-	select CLOCKSOURCE_VALIDATE_LAST_CYCLE
 	select CLOCKSOURCE_WATCHDOG
 	# Word-size accesses may read uninitialized data past the trailing \0
 	# in strings and cause false KMSAN reports.

kernel/time/Kconfig

@@ -17,11 +17,6 @@ config ARCH_CLOCKSOURCE_DATA
 config ARCH_CLOCKSOURCE_INIT
 	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

@@ -30,7 +30,6 @@ static inline void timekeeping_inc_mg_floor_swaps(void)
 
 #endif
 
-#ifdef CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE
 static inline u64 clocksource_delta(u64 now, u64 last, u64 mask)
 {
 	u64 ret = (now - last) & mask;
@@ -41,12 +40,6 @@ static inline u64 clocksource_delta(u64 now, u64 last, u64 mask)
 	 */
 	return ret & ~(mask >> 1) ? 0 : ret;
 }
-#else
-static inline u64 clocksource_delta(u64 now, u64 last, u64 mask)
-{
-	return (now - last) & mask;
-}
-#endif
 
 /* Semi public for serialization of non timekeeper VDSO updates. */
 unsigned long timekeeper_lock_irqsave(void);