vdso/math64: Add and use __iter_div64_u64_rem()

The vDSO basetimes for CLOCK_MONOTONIC and CLOCK_BOOTTIME are stored in
the scaled nanoseconds of tkr_mono, so normalising them requires a
division by NSEC_PER_SEC << shift.  That divisor does not fit the u32
parameter of __iter_div_u64_rem(), so update_vdso_time_data() open-codes
the same iterative division twice.

Repeated subtraction is the appropriate form at these two sites because
the quotient never exceeds one.  accumulate_nsecs_to_secs() keeps
xtime_nsec below one scaled second, and the offset added to it is a
normalised timespec64 fraction, so the dividend stays below twice the
divisor.  That bound holds on every architecture, which matters more here
than the cost of a division on any particular one.

Add __iter_div64_u64_rem(), the u64-divisor counterpart of
__iter_div_u64_rem(), and use it at both sites.  Return the quotient as a
u32 for consistency with the u32-divisor variant; the bound above leaves
no use for a wider type.

Store the remainder directly into the basetime, as the coarse clocks
already do.  The CLOCK_BOOTTIME copy of the CLOCK_MONOTONIC values then
takes both fields from the same place.

Folding the two open-coded loops into one inlined helper removes 16 bytes
of vsyscall.o text on x86-64 with gcc 13.

No functional change.

Suggested-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://patch.msgid.link/20261002153104.1347978-3-zhanxusheng@xiaomi.com

1 files changed, 20 insertions, 0 deletions

diff --git a/include/vdso/math64.h b/include/vdso/math64.h
index 3c08909e742b..6f1e311cfdbd 100644
--- a/include/vdso/math64.h
+++ b/include/vdso/math64.h
@@ -24,6 +24,26 @@ static __always_inline u32 __iter_div_u64_rem(u64 dividend, u32 divisor, u64 *re
 	return ret;
 }
 
+static __always_inline u32 __iter_div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
+{
+	u32 ret = 0;
+
+	while (dividend >= divisor) {
+		/*
+		 * Prevent the compiler from optimising this loop into a
+		 * modulo operation.
+		 */
+		OPTIMIZER_HIDE_VAR(dividend);
+
+		dividend -= divisor;
+		ret++;
+	}
+
+	*remainder = dividend;
+
+	return ret;
+}
+
 #if defined(CONFIG_ARCH_SUPPORTS_INT128) && defined(__SIZEOF_INT128__)
 
 #ifndef mul_u64_u32_add_u64_shr