From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
nathanl@linux.ibm.com, arnd@arndb.de, tglx@linutronix.de,
vincenzo.frascino@arm.com, luto@kernel.org
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
x86@kernel.org
Subject: [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation
Date: Thu, 16 Jan 2020 17:58:34 +0000 (UTC) [thread overview]
Message-ID: <c8ce9baaef0dc7273e4bcc31f353b17b655113d1.1579196675.git.christophe.leroy@c-s.fr> (raw)
In-Reply-To: <cover.1579196675.git.christophe.leroy@c-s.fr>
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 nul. GCC makes reversed assumptions considering
the shift to be likely >= 32 and the upper part to be like not nul.
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 with an &= 31, it still considers
the shift as likely >= 32.
Define a vdso_shift_ns() macro that can be overriden by
arches.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
lib/vdso/gettimeofday.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index 724b45c3e8ac..9ba92058cfd7 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, unsigned long shift)
+{
+ return ns >> shift;
+}
+#endif
+
#ifndef __arch_vdso_hres_capable
static inline bool __arch_vdso_hres_capable(void)
{
@@ -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)));
--
2.13.3
next prev parent reply other threads:[~2020-01-16 17:58 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-16 17:58 [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 01/11] powerpc/64: Don't provide time functions in compat VDSO32 Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 02/11] powerpc/vdso: Switch VDSO to generic C implementation Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 03/11] lib: vdso: only read hrtimer_res when needed in __cvdso_clock_getres() Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 04/11] powerpc/vdso: simplify __get_datapage() Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 05/11] lib: vdso: allow arches to provide vdso data pointer Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 06/11] powerpc/vdso: provide inline alternative to __get_datapage() Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 07/11] powerpc/vdso: provide vdso data pointer from the ASM caller Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 08/11] lib: vdso: allow fixed clock mode Christophe Leroy
2020-01-16 20:13 ` Thomas Gleixner
2020-01-16 20:19 ` Andy Lutomirski
2020-01-16 21:07 ` Thomas Gleixner
2020-01-16 17:58 ` [RFC PATCH v4 09/11] powerpc/vdso: override __arch_vdso_capable() Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy [this message]
2020-01-16 19:47 ` [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation Andy Lutomirski
2020-01-16 19:57 ` Thomas Gleixner
2020-01-16 20:20 ` Andy Lutomirski
2020-01-29 7:14 ` Thomas Gleixner
2020-01-29 7:26 ` Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 11/11] powerpc/32: provide vdso_shift_ns() Christophe Leroy
2020-01-17 8:58 ` [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation Segher Boessenkool
2020-01-17 9:26 ` Christophe Leroy
2020-01-20 14:56 ` Christophe Leroy
2020-01-20 15:19 ` Segher Boessenkool
2020-01-20 17:08 ` Christophe Leroy
2020-01-20 17:27 ` Segher Boessenkool
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