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authorChristophe Leroy <christophe.leroy@c-s.fr>2020-02-07 15:39:03 +0300
committerThomas Gleixner <tglx@linutronix.de>2020-02-17 22:12:18 +0300
commit8345228ccf31f94e3ff7ec5458ac7cc13cb323fa (patch)
treeb471dcfc00817263fb225dc1ae66b235fd36b316 /lib/vdso
parentae12e08539de6717502c2f9f83bd60df939b5c08 (diff)
downloadlinux-8345228ccf31f94e3ff7ec5458ac7cc13cb323fa.tar.xz
lib/vdso: Allow architectures to override the ns shift operation
On powerpc/32, GCC (8.1) generates pretty bad code for the ns >>= vd->shift operation taking into account that the shift is always <= 32 and the upper part of the result is likely to be zero. GCC makes reversed assumptions considering the shift to be likely >= 32 and the upper part to be like not zero. unsigned long long shift(unsigned long long x, unsigned char s) { return x >> s; } results in: 00000018 <shift>: 18: 35 25 ff e0 addic. r9,r5,-32 1c: 41 80 00 10 blt 2c <shift+0x14> 20: 7c 64 4c 30 srw r4,r3,r9 24: 38 60 00 00 li r3,0 28: 4e 80 00 20 blr 2c: 54 69 08 3c rlwinm r9,r3,1,0,30 30: 21 45 00 1f subfic r10,r5,31 34: 7c 84 2c 30 srw r4,r4,r5 38: 7d 29 50 30 slw r9,r9,r10 3c: 7c 63 2c 30 srw r3,r3,r5 40: 7d 24 23 78 or r4,r9,r4 44: 4e 80 00 20 blr Even when forcing the shift to be smaller than 32 with an &= 31, it still considers the shift as likely >= 32. Move the default shift implementation into an inline which can be redefined in architecture code via a macro. [ tglx: Made the shift argument u32 and removed the __arch prefix ] Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com> Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com> Link: https://lore.kernel.org/r/b3d449de856982ed060a71e6ace8eeca4654e685.1580399657.git.christophe.leroy@c-s.fr Link: https://lkml.kernel.org/r/20200207124403.857649978@linutronix.de
Diffstat (limited to 'lib/vdso')
-rw-r--r--lib/vdso/gettimeofday.c11
1 files changed, 9 insertions, 2 deletions
diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index 8eb6d1e9a8ff..b95aef97501e 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -39,6 +39,13 @@ u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
}
#endif
+#ifndef vdso_shift_ns
+static __always_inline u64 vdso_shift_ns(u64 ns, u32 shift)
+{
+ return ns >> shift;
+}
+#endif
+
#ifndef __arch_vdso_hres_capable
static inline bool __arch_vdso_hres_capable(void)
{
@@ -80,7 +87,7 @@ static int do_hres_timens(const struct vdso_data *vdns, clockid_t clk,
ns = vdso_ts->nsec;
last = vd->cycle_last;
ns += vdso_calc_delta(cycles, last, vd->mask, vd->mult);
- ns >>= vd->shift;
+ ns = vdso_shift_ns(ns, vd->shift);
sec = vdso_ts->sec;
} while (unlikely(vdso_read_retry(vd, seq)));
@@ -148,7 +155,7 @@ static __always_inline int do_hres(const struct vdso_data *vd, clockid_t clk,
ns = vdso_ts->nsec;
last = vd->cycle_last;
ns += vdso_calc_delta(cycles, last, vd->mask, vd->mult);
- ns >>= vd->shift;
+ ns = vdso_shift_ns(ns, vd->shift);
sec = vdso_ts->sec;
} while (unlikely(vdso_read_retry(vd, seq)));