[PATCH AUTOSEL 5.4 2/2] timekeeping: Always check for negative motion

[Sasha Levin]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

[ Upstream commit c163e40af9b2331b2c629fd4ec8b703ed4d4ae39 ]

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 135225a363ae 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@xxxxxxxxxxxxx>
Acked-by: John Stultz <jstultz@xxxxxxxxxx>
Link: https://lore.kernel.org/all/20241031120328.599430157@xxxxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/x86/Kconfig                   | 1 -
 kernel/time/Kconfig                | 5 -----
 kernel/time/timekeeping_internal.h | 7 -------
 3 files changed, 13 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index df0a3a1b08ae0..a3e7f75a8cfa8 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -99,7 +99,6 @@ config X86
 	select ARCH_WANTS_THP_SWAP		if X86_64
 	select BUILDTIME_EXTABLE_SORT
 	select CLKEVT_I8253
-	select CLOCKSOURCE_VALIDATE_LAST_CYCLE
 	select CLOCKSOURCE_WATCHDOG
 	select DCACHE_WORD_ACCESS
 	select EDAC_ATOMIC_SCRUB
diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index fcc42353f1253..4b9a8653a6327 100644
--- a/kernel/time/Kconfig
+++ b/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
diff --git a/kernel/time/timekeeping_internal.h b/kernel/time/timekeeping_internal.h
index bcbb52db22565..d7f99e69bce4b 100644
--- a/kernel/time/timekeeping_internal.h
+++ b/kernel/time/timekeeping_internal.h
@@ -13,7 +13,6 @@ extern void tk_debug_account_sleep_time(const struct timespec64 *t);
 #define tk_debug_account_sleep_time(x)
 #endif
 
-#ifdef CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE
 static inline u64 clocksource_delta(u64 now, u64 last, u64 mask)
 {
 	u64 ret = (now - last) & mask;
@@ -24,11 +23,5 @@ 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
 
 #endif /* _TIMEKEEPING_INTERNAL_H */
-- 
2.43.0