Re: [PATCH] Linux: Use 32-bit vDSO for clock_gettime, gettimeofday, time (bug 28071)

[Florian Weimer via Libc-alpha <libc-alpha@sourceware.org>]

* Adhemerval Zanella:

> On 12/07/2021 07:40, Florian Weimer wrote:
>> * Adhemerval Zanella:
>> 
>>> Using the test (a slight modified one from the bug report):
>>>
>>> --
>>> #include <time.h>
>>> #include <stdio.h>
>>> #include <errno.h>
>>>
>>> int
>>> main (void)
>>> {
>>>   struct timespec ts;
>>>   errno = 0;
>>>   clock_gettime (CLOCK_REALTIME, &ts);
>>>   printf ("errno = %m (%d)\n", errno);
>>>   errno = 0;
>>>   clock_gettime (CLOCK_MONOTONIC, &ts);
>>>   printf ("errno = %m (%d)\n", errno);
>>> }
>>> --
>>>
>>> I see no syscall on 5.11 kernel, only a clock_gettime (CLOCK_MONOTONIC)
>>> on the 4.4 and a clock_gettime_time64 plus a clock_gettime on the 3.10.
>> 
>> This still introduces a severe performance regression on older kernels.
>> It may well make some exsting 32-bit applications unusable until the
>> kernel is upgraded.  I'm not sure if this is a good idea.
>> 
>> At least I can see that the clobbering of errno is gone.
>
> Yes and this how we initially decided to provide 64-bit time_t support,
> where 32-bit implementations are done on top of 64-bit ones.  I am not
> very fond of starting to adding 32-bit specific implementations, the
> complexity to fix some specific cases do not really pay in the long
> term imho.  
>
> We might also add back the global time64 internal variable that indicates
> if the kernel supports 64-bit (removed by 9465c3a9fb557), but it also has
> its ow issues (live migration like CRIU).

Hmm.  I see.  Let's fix the ENOSYS issue then because it breaks OpenJDK.

I'm a bit surprised that we still see the extra syscalls with your
patch, but I suppose that's just the way the INTERNAL_VSYSCALL_CALL
macro works.

Regarding the actual patch, there are a few missing spaces before
parenthesis:

+  int (*vdso_time64)(clockid_t clock_id, struct __timespec64 *tp)
+    = GLRO(dl_vdso_clock_gettime64);
+  int (*vdso_time)(clockid_t clock_id, struct timespec *tp)
+    = GLRO(dl_vdso_clock_gettime);

Thanks,
Florian