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@linutronix.de>
---
 arch/x86/Kconfig                   |    1 -
 kernel/time/Kconfig                |    5 -----
 kernel/time/timekeeping_internal.h |    7 -------
 3 files changed, 13 deletions(-)

--- a/arch/x86/Kconfig
+++ b/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.
--- 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
--- a/kernel/time/timekeeping_internal.h
+++ b/kernel/time/timekeeping_internal.h
@@ -30,7 +30,6 @@ static inline void timekeeping_inc_mg_fl
 
 #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
 	 */
 	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);

On Thu, Oct 31, 2024 at 5:04 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> 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@linutronix.de>

Acked-by: John Stultz <jstultz@google.com>

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     c163e40af9b2331b2c629fd4ec8b703ed4d4ae39
Gitweb:        https://git.kernel.org/tip/c163e40af9b2331b2c629fd4ec8b703ed4d4ae39
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Thu, 31 Oct 2024 13:04:08 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Sat, 02 Nov 2024 10:14:31 +01:00

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

---
 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 2852fcd..53a5eda 100644
--- a/arch/x86/Kconfig
+++ b/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.
diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 8ebb6d5..b0b97a6 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 b3dca83..63e600e 100644
--- a/kernel/time/timekeeping_internal.h
+++ b/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);