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

[Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org>]

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).