Re: riscv64: ld.so hardware capabilities / V extension 1.0

[Florian Weimer <fweimer@redhat.com>]

* Mathieu Malaterre:

> Dear ld.so maintainers,
>
> I see that from time to time subdirectories are added to the search
> path of ld.so. For example I see that commit 45ff34638f0 added the
> sub-directory `avx512_1`.
>
> I tried to make sense of the (outdated?) documentation from:
>
> * https://man7.org/linux/man-pages/man8/ld.so.8.html
>
> but I failed to understand when new subdirectories are being added to ld.so ?

The old mechanism with AT_PLATFORM did not work.  The new mechanism is
glibc-hwcaps, where capabilities are ordered, ideally with each
capability being a superset of the previous one.  You only need to
supply one file, dl-hwcaps-subdirs.c, that defines _dl_hwcaps_subdirs
and _dl_hwcaps_subdirs_active.  This commit provides an example of how
to do this:

commit fdf8fbca455ca3ef57235bde907bcc6a624ac5aa
Author: Florian Weimer <fweimer@redhat.com>
Date:   Thu Dec 10 13:51:18 2020 +0100

    s390x: Add glibc-hwcaps support
    
    Subdirectories z13, z14, z15 can be selected, mostly based on the
    level of support for vector instructions.
    
    Co-Authored-By: Stefan Liebler <stli@linux.ibm.com>

Ideally, you add similar tests to what's included in that commit.

I'd also strongly recommend to document the glibc-hwcaps levels in the
psABI supplement because they need to be consistent across toolchain
components.

If you want to generate nice diagnostics when glibc itself is built with
a higher ISA level, you'll have to add your own dl-hwcap-check.h

Thanks,
Florian