gettimeofday / vdso / aarch64 question
Commit Message
I was wondering if someone could look at my proposed getimeofday.c
implementation for aarch64 and help me figure out what I am doing
wrong. I am trying to use the VDSO interface to speed it up and
it looks like that should be supported starting with Linux 2.6.39
(according to sysdeps/unix/sysv/linux/aarch64/init-first.c) but
when I include this gettimeofday.c in sysdeps/unix/sysv/linux/aarch64
I get a link failure with undefined references to __GI___gettimeofday.
I think something is wrong with my use of the various symbol macros,
probably with libc_ifunc_hidden or __hidden_ver1, but I can't seem to
figure out what the right incantation is.
Steve Ellcey
sellcey@cavium.com
Comments
On Mai 09 2018, Steve Ellcey <sellcey@cavium.com> wrote:
> +libc_ifunc_hidden (__redirect___gettimeofday, __gettimeofday,
> + vdso_gettimeofday ?: (void *) __gettimeofday_syscall)
> +# undef libc_hidden_def
> +# define libc_hidden_def(name) \
> + __hidden_ver1 (__gettimeofday_syscall, __GI___gettimeofday, \
> + __gettimeofday_syscall);
> +weak_alias (__gettimeofday, gettimeofday)
> +libc_hidden_weak (gettimeofday)
You are (re)defining libc_hidden_def, but don't use it.
Andreas.
On Thu, 2018-05-10 at 09:42 +0200, Andreas Schwab wrote:
> On Mai 09 2018, Steve Ellcey <sellcey@cavium.com> wrote:
>
> >
> > +libc_ifunc_hidden (__redirect___gettimeofday, __gettimeofday,
> > + vdso_gettimeofday ?: (void *)
> > __gettimeofday_syscall)
> > +# undef libc_hidden_def
> > +# define libc_hidden_def(name) \
> > + __hidden_ver1 (__gettimeofday_syscall, __GI___gettimeofday, \
> > + __gettimeofday_syscall);
> > +weak_alias (__gettimeofday, gettimeofday)
> > +libc_hidden_weak (gettimeofday)
> You are (re)defining libc_hidden_def, but don't use it.
>
> Andreas.
Argh. Staring at it too long to see it. I have it building now,
I will submit a patch once the testsuite finishes.
Steve Ellcey
On 10/05/2018 13:22, Steve Ellcey wrote:
> On Thu, 2018-05-10 at 09:42 +0200, Andreas Schwab wrote:
>> On Mai 09 2018, Steve Ellcey <sellcey@cavium.com> wrote:
>>
>>>
>>> +libc_ifunc_hidden (__redirect___gettimeofday, __gettimeofday,
>>> + vdso_gettimeofday ?: (void *)
>>> __gettimeofday_syscall)
>>> +# undef libc_hidden_def
>>> +# define libc_hidden_def(name) \
>>> + __hidden_ver1 (__gettimeofday_syscall, __GI___gettimeofday, \
>>> + __gettimeofday_syscall);
>>> +weak_alias (__gettimeofday, gettimeofday)
>>> +libc_hidden_weak (gettimeofday)
>> You are (re)defining libc_hidden_def, but don't use it.
>>
>> Andreas.
>
> Argh. Staring at it too long to see it. I have it building now,
> I will submit a patch once the testsuite finishes.
>
> Steve Ellcey
>
Keep in mind that different that x86 and powerpc implementations, where
the vDSO symbol does not fail; the arm64 vDSO implements a syscall
fallback in case of underlying hardware requires an out-of-line counter
access (arch_timer_enable_workaround).
Using a ifunc accessors to call vDSO directly will result in a slight
different semantic since generic implementation (kernel/time/time.c)
might return EFAULT in some cases (which won't be handled by ifunc
implementation). This should not be an issue since POSIX [1] defines
no error code should reserved for the symbol, but it might trigger
some test in LTP.
I recommend you document it on commit log with the rationale to use
ifunc instead of usual INLINE_VSYSCALL.
[1] http://pubs.opengroup.org/onlinepubs/9699919799/
@@ -0,0 +1,66 @@
+/* Copyright (C) 2018 Free Software Foundation, Inc.
+
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public License as
+ published by the Free Software Foundation; either version 2.1 of the
+ License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <http://www.gnu.org/licenses/>. */
+
+/* Get the current time of day and timezone information,
+ putting it into *tv and *tz. If tz is null, *tz is not filled.
+ Returns 0 on success, -1 on errors. */
+
+#ifdef SHARED
+
+# define __gettimeofday __redirect___gettimeofday
+# include <sys/time.h>
+# undef __gettimeofday
+# include <dl-vdso.h>
+static int
+__gettimeofday_syscall (struct timeval *tv, struct timezone *tz)
+{
+ /* Should this be INLINE_SYSCALL or INLINE_VSYSCALL? */
+ return INLINE_SYSCALL (gettimeofday, 2, tv, tz);
+}
+/* PREPARE_VERSION will need an __LP64__ ifdef when ILP32 support
+ goes in. See _libc_vdso_platform_setup in
+ sysdeps/unix/sysv/linux/aarch64/init-first.c. */
+
+# undef INIT_ARCH
+# define INIT_ARCH() \
+ PREPARE_VERSION (linux_version, "LINUX_2.6.39", 123718537); \
+ void *vdso_gettimeofday = \
+ _dl_vdso_vsym ("__kernel_gettimeofday", &linux_version);
+
+libc_ifunc_hidden (__redirect___gettimeofday, __gettimeofday,
+ vdso_gettimeofday ?: (void *) __gettimeofday_syscall)
+# undef libc_hidden_def
+# define libc_hidden_def(name) \
+ __hidden_ver1 (__gettimeofday_syscall, __GI___gettimeofday, \
+ __gettimeofday_syscall);
+weak_alias (__gettimeofday, gettimeofday)
+libc_hidden_weak (gettimeofday)
+
+#else
+
+# include <sys/time.h>
+# include <sysdep.h>
+int
+__gettimeofday (struct timeval *tv, struct timezone *tz)
+{
+ return INLINE_SYSCALL (gettimeofday, 2, tv, tz);
+}
+libc_hidden_def (__gettimeofday)
+weak_alias (__gettimeofday, gettimeofday)
+libc_hidden_weak (gettimeofday)
+#endif