linux-kernel - [patch 2/2] timekeeping: Always check for negative motion

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Message-ID: <20241031120328.599430157@linutronix.de>
Date: Thu, 31 Oct 2024 13:04:08 +0100 (CET)
From: Thomas Gleixner <tglx@...utronix.de>
To: LKML <linux-kernel@...r.kernel.org>
Cc: John Stultz <jstultz@...gle.com>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>,
 Frederic Weisbecker <frederic@...nel.org>,
 Stephen Boyd <sboyd@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>
Subject: [patch 2/2] 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@...utronix.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);