Re: s390x: Diagnose missing VXE at run time if required at build time

[Stefan Liebler <stli@linux.ibm.com>]

On 08/04/2021 17:30, Florian Weimer wrote:
> * Stefan Liebler:
> 
>> I've just had a quick look to dl_platform_init for other architectures
>> to check if there are similar checks. But I haven't found those there.
>> Are there similar checks for other architectures at a different place?
> 
> There is a check in sysdeps/x86/dl-prop.h, but that's different because
> it looks at ELF data, not build flags.
> 
>> Is there a special reason for this check beyond giving the user an error
>> message instead of crashing with SIGILL?
> 
> No, there isn't.  It's merely about the diagnostic.
> 
>> The user will get the error message if build with -march=z14 and running
>> on z13. If this binary is running on zEC12, it just crashes with SIGILL
>> as there is a z13 vector instructions before dl_platform_init. Currently
>> it works on z13, but who knows if this does not change if build with a
>> different gcc version?
> 
> To be honest, this change is mostly driven by downstream requirements.
> Our zEC12 hardware is powered off, but the z13 hardware is not, so there
> is some space for confusion.
> 
> I'm going to post a similar patch for POWER9 if I can get it to work.
> There we actually hit the issue you mention: some of them min/max
> instructions are actually used early during startup.  I don't know yet
> if that's before or after the point I'm patching here for s390x.
> 
>> The __LONG_DOUBLE_VX__ macro is quite new and the first gcc release will
>> be gcc 11. But there is also the __ARCH__ macro. If defined by the used
>> gcc, it is set to the architecture level determined by -march=xyz
>> (z15=13; z14=12; z13=11; ...).
>> See gcc commit "S/390: Rename __S390_ARCH_LEVEL__ to __ARCH__."
>> https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=4727e06bb7c047a10aa502c829b7e4b519d8082b
>> If I remember correctly, this was introduced with gcc 7 or 8.
>>
>> Would it make sense to add such a check also if build with -march=z15?:
>> #if defined __ARCH__
>> # if __ARCH__ >= 13
>>   if (!(GLRO(dl_hwcap) & HWCAP_S390_VXRS_EXT2))
>>     _dl_fatal_printf ("\
>> Fatal glibc error: CPU lacks VXRS_EXT2 support (z15 or later required)\n");
>> # elif __ARCH__ >= 12
>>   if (!(GLRO(dl_hwcap) & HWCAP_S390_VXE))
>>     _dl_fatal_printf ("\
>> Fatal glibc error: CPU lacks VXE support (z14 or later required)\n");
>> #endif
> 
> This is fine with me too.
Okay. Then I would prefer the __ARCH__ variant.

Thanks,
Stefan