linux-kernel - [PATCH V2 16/19] timekeeping: Prepare timekeeping_cycles_to

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Date: Mon, 25 Mar 2024 08:40:20 +0200
From: Adrian Hunter <adrian.hunter@...el.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	"Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Vincenzo Frascino <vincenzo.frascino@....com>,
	John Stultz <jstultz@...gle.com>,
	Stephen Boyd <sboyd@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Arnd Bergmann <arnd@...db.de>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org,
	linux-s390@...r.kernel.org
Subject: [PATCH V2 16/19] 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@...utronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
---
 kernel/time/timekeeping.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 749387f22f0f..d17484082e2c 100644
--- 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;
 }
-- 
2.34.1