Re: [PATCH, resend] Add x86 32 bit vDSO time function support

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Jan 12, 2015 at 6:28 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Mon, Jan 12, 2015 at 5:51 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Tue, Jan 6, 2015 at 6:35 AM, Adhemerval Zanella
>> <azanella@linux.vnet.ibm.com> wrote:
>>> Ping.
>>>
>>> On 09-12-2014 13:32, Adhemerval Zanella wrote:
>>>> Linux 3.15 adds support for clock_gettime, gettimeofday, and time vDSO
>>>> (commit id 37c975545ec63320789962bf307f000f08fabd48).  This patch adds
>>>> GLIBC supports to use such symbol when they are avaiable.
>>>>
>>>> Along with x86 vDSO support, this patch cleanup x86_64 code by moving
>>>> all common code to x86 common folder.  Only init-first.c is different
>>>> between implementations.
>>>>
>>>> Tested on x86 and x86_64 with a 3.5.0 and a 3.16 kernel.
>>>>
>>>> --
>>>>
>>>> 2014-12-09  Adhemerval Zanella  <azanella@linux.vnet.ibm.com>
>>>>           Stefani Seibold  <stefani@seibold.net>
>>>>
>>>>       * sysdeps/unix/sysv/linux/x86_64/Makefile [$(subdir) = elf]
>>>>       (sysdep_routines): Add dl-vdso here, ...
>>>>       * sysdeps/unix/sysv/linux/x86/Makefile [$(subdir) = elf]
>>>>       (sysdep_routines): ... not here.
>>>>       * sysdeps/unix/sysv/linux/i386/gettimeofday.c: New file: set syscall
>>>>       fallback when vDSO is not presented.
>>>>       * sysdeps/unix/sysv/linux/x86_64/init-first.c (__vdso_clock_gettime):
>>>>       Define with libc_hidden_proto/libc_hidden_data_def definitions.
>>>>       (_libc_vdso_platform_setup): Rename to __vdso_platform_setup.
>>>>       * sysdeps/unix/sysv/linux/i386/init-first.c: New file: likewise.
>>>>       * sysdeps/unix/sysv/linux/i386/time.c: New file: likewise.
>>>>       * sysdeps/unix/sysv/linux/x86_64/bits/libc-vdso.h: Moved to ...
>>>>       * sysdeps/unix/sysv/linux/x86/libc-vdso.h: ... here.
>>>>       * sysdeps/unix/sysv/linux/x86_64/clock_gettime.c: Move to ...
>>>>       * sysdeps/unix/sysv/linux/x86/clock_gettime.c: ... here.
>>>>       * sysdeps/unix/sysv/linux/x86_64/gettimeofday.c: Move to ...
>>>>       * sysdeps/unix/sysv/linux/x86/gettimeofday.c: ... here.  Also added
>>>>       fallback configurable symbol when vDSO is not available.
>>>>       * sysdeps/unix/sysv/linux/x86_64/time.c: Move to ...
>>>>       * sysdeps/unix/sysv/linux/x86/time.c: ... here.  Also refactored to
>>>>       be able to redefine fallback symbol when vDSO is not available.
>>>>       * sysdeps/unix/sysv/linux/x86_64/timespec_get.c: Move to ...
>>>>       * sysdeps/unix/sysv/linux/x86/timespec_get.c: ... here.
>>>>
>>>> --
>>>>
>>
>>>> +# include <dl-vdso.h>
>>>> +# include <errno.h>
>>>> +
>>>> +/* If the vDSO is not available we fall back on the syscall.  */
>>>> +static int
>>>> +__gettimeofday_syscall (struct timeval *tv, struct timezone *tz)
>>>> +{
>>>> +  return INLINE_SYSCALL (gettimeofday, 2, tv, tz);
>>>> +}
>>>> +# define GETTIMEOFAY_FALLBACK (void*) (&__gettimeofday_syscall)
>>>> +# undef libc_ifunc_hidden_def
>>>> +# define libc_ifunc_hidden_def(name)  \
>>>> +  libc_ifunc_hidden_def1 (__GI_##name, __gettimeofday_syscall)
>>
>> Why do you define __GI___gettimeofday to __gettimeofday_syscall
>> instead of the ifunc version? The x86-64 ifunc  works fine.
>>
>> --
>> H.J.
>
> Why isn't __GI_time IFUNC
>
> 5822: 000a3360    21 FUNC    LOCAL  DEFAULT   13 __GI_time
>

The key to call local IFUNC function is that we HAVE to tell GCC
it is a global function, not local so that it will go through PLT.
Otherwise GCC will perform optimization which will break local
IFUNC.  I will see what I can do.


-- 
H.J.