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

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

On 06/04/2021 17:55, Florian Weimer wrote:
> It turns out that if glibc is built with -march=z14, it is still
> possible to run up to dl_platform_init on z13 CPUs.  This means we can
> add a diagnostic to glibc, notifying the user that the CPU is too old.
> 
> The check is based on the way GCC sets the __LONG_DOUBLE_VX__
> preprocessor macro:
> 
>   s390_def_or_undef_macro (
>       pfile,
>       [] (const struct cl_target_option *opts) { return TARGET_VXE_P (opts); },
>       old_opts, opts, "__LONG_DOUBLE_VX__", "__LONG_DOUBLE_VX__");
> 
> 
> diff --git a/sysdeps/s390/s390-64/dl-machine.h b/sysdeps/s390/s390-64/dl-machine.h
> index 543361c83637c071..deaf37951206fb7c 100644
> --- a/sysdeps/s390/s390-64/dl-machine.h
> +++ b/sysdeps/s390/s390-64/dl-machine.h
> @@ -235,6 +235,12 @@ _dl_start_user:\n\
>  static inline void __attribute__ ((unused))
>  dl_platform_init (void)
>  {
> +#ifdef __LONG_DOUBLE_VX__
> +  if (!(GLRO(dl_hwcap) & HWCAP_S390_VXE))
> +    _dl_fatal_printf ("\
> +Fatal glibc error: CPU lacks VXE support (z14 or later required)\n");
> +#endif
> +
>    if (GLRO(dl_platform) != NULL && *GLRO(dl_platform) == '\0')
>      /* Avoid an empty string which would disturb us.  */
>      GLRO(dl_platform) = NULL;
> 

Hi Florian,

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?

Is there a special reason for this check beyond giving the user an error
message instead of crashing with SIGILL?

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?

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

There are also configure checks which checks if arch-level specific
instructions are available at build-time (but not for all levels):
z15: HAVE_S390_MIN_ARCH13_ZARCH_ASM_SUPPORT
z13: HAVE_S390_MIN_Z13_ZARCH_ASM_SUPPORT
z196: HAVE_S390_MIN_Z196_ZARCH_ASM_SUPPORT
z10: HAVE_S390_MIN_Z10_ZARCH_ASM_SUPPORT

Thanks,
Stefan