[PATCH v3 1/3] vdso/math64: Add and use __iter_div64_u64_rem()

[Zhan Xusheng <zhanxusheng1024@gmail.com>]

The vDSO basetimes for CLOCK_MONOTONIC and CLOCK_BOOTTIME are kept in the
scaled nanoseconds of tkr_mono, so normalising them means dividing by
NSEC_PER_SEC << shift, which does not fit the u32 divisor of
__iter_div_u64_rem().

update_vdso_time_data() therefore open-codes the iterative division twice.
Turning the loops into a plain modulo is not an option either, as the vDSO
has no 64-bit division helpers on 32-bit.

Add __iter_div64_u64_rem(), the u64-divisor counterpart of
__iter_div_u64_rem(), and use it for both.  The remainder goes straight
into the basetime as the coarse clocks already do, which also makes the
copy of the CLOCK_MONOTONIC values for CLOCK_BOOTTIME take both of them
from the same place.

The quotient is a u32 like the u32-divisor version returns.  A loop that
subtracts only makes sense when it iterates a handful of times, and it
keeps 32-bit from carrying the counter in a register pair: vsyscall.o
loses 16 bytes of text on x86-64 and the loop drops from 36 to 28
instructions on 32-bit gcc.

The barrier keeps the value in a register rather than offering a memory
alternative like __iter_div_u64_rem() does.  Both forms stop the loop
becoming a division, but clang takes the memory alternative and spills
inside the loop, on 64-bit as well.  Converting __iter_div_u64_rem() the
same way is worth 64 bytes of vdso64 text and 112 of vdso32 built with
clang 18, since it sits on the clock_gettime() path through
vdso_set_timespec(); that is left for a separate patch as it also has
users outside the vDSO.

No functional change.

Suggested-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
---
 include/vdso/math64.h  | 22 ++++++++++++++++++++++
 kernel/time/vsyscall.c | 18 +++++++-----------
 2 files changed, 29 insertions(+), 11 deletions(-)

diff --git a/include/vdso/math64.h b/include/vdso/math64.h
index 22ae212f8b28..02abdf6e82ed 100644
--- a/include/vdso/math64.h
+++ b/include/vdso/math64.h
@@ -21,6 +21,28 @@ __iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
 	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.  Keep the value in a register, as clang
+		 * spills it when offered a memory alternative.
+		 */
+		asm("" : "=r"(dividend) : "0"(dividend));
+
+		dividend -= divisor;
+		ret++;
+	}
+
+	*remainder = dividend;
+
+	return ret;
+}
+
 #if defined(CONFIG_ARCH_SUPPORTS_INT128) && defined(__SIZEOF_INT128__)
 
 #ifndef mul_u64_u32_add_u64_shr
diff --git a/kernel/time/vsyscall.c b/kernel/time/vsyscall.c
index aa59919b8f2c..993258d5f1e7 100644
--- a/kernel/time/vsyscall.c
+++ b/kernel/time/vsyscall.c
@@ -41,14 +41,13 @@ static inline void update_vdso_time_data(struct vdso_time_data *vdata, struct ti
 
 	nsec = tk->tkr_mono.xtime_nsec;
 	nsec += ((u64)tk->wall_to_monotonic.tv_nsec << tk->tkr_mono.shift);
-	while (nsec >= (((u64)NSEC_PER_SEC) << tk->tkr_mono.shift)) {
-		nsec -= (((u64)NSEC_PER_SEC) << tk->tkr_mono.shift);
-		vdso_ts->sec++;
-	}
-	vdso_ts->nsec	= nsec;
+	vdso_ts->sec	+= __iter_div64_u64_rem(nsec,
+				(u64)NSEC_PER_SEC << tk->tkr_mono.shift,
+				&vdso_ts->nsec);
 
 	/* Copy MONOTONIC time for BOOTTIME */
 	sec	= vdso_ts->sec;
+	nsec	= vdso_ts->nsec;
 	/* Add the boot offset */
 	sec	+= tk->monotonic_to_boot.tv_sec;
 	nsec	+= (u64)tk->monotonic_to_boot.tv_nsec << tk->tkr_mono.shift;
@@ -56,12 +55,9 @@ static inline void update_vdso_time_data(struct vdso_time_data *vdata, struct ti
 	/* CLOCK_BOOTTIME */
 	vdso_ts		= &vc[CS_HRES_COARSE].basetime[CLOCK_BOOTTIME];
 	vdso_ts->sec	= sec;
-
-	while (nsec >= (((u64)NSEC_PER_SEC) << tk->tkr_mono.shift)) {
-		nsec -= (((u64)NSEC_PER_SEC) << tk->tkr_mono.shift);
-		vdso_ts->sec++;
-	}
-	vdso_ts->nsec	= nsec;
+	vdso_ts->sec	+= __iter_div64_u64_rem(nsec,
+				(u64)NSEC_PER_SEC << tk->tkr_mono.shift,
+				&vdso_ts->nsec);
 
 	/* CLOCK_MONOTONIC_RAW */
 	vdso_ts		= &vc[CS_RAW].basetime[CLOCK_MONOTONIC_RAW];
-- 
2.43.0