Re: [RFC PATCH 00/19] riscv: ifunc support with optimized mem*/str*/cpu_relax routines

[DJ Delorie <dj@redhat.com>]

Adhemerval Zanella Netto <adhemerval.zanella@linaro.org> writes:
> So now we have 3 different proposal mechanism to provide implementation runtime 
> selection on riscv:
>
>   1. The sysdep mechanism to select optimized routines based on compiler/ABI
>      done at build time.  It is the current mechanism and it is also used
>      on rvv routines [1].
>
>   2. A ifunc one using a new riscv syscall to query the kernel the required 
>      information.
>
>   3. Another ifunc one using riscv specific environment variable.

I'm also going to oppose #3 on principles.  We've been removing the use
of environment variables for tuning, in favor of tunables.

If we have a way to auto-detect the best implementation without relying
on the user, that's my preference.  Users are unreliable and require
documentation.  The compiler likely doesn't have access to the
hardware[*], so must rely on the user.  Thus, my preference is #2 - the
kernel has access to the hardware and its device tree, and can tell the
userspace what capabilities are available.

I would not be opposed to a tunable that overrides the autodetection; we
have something similar for x86.  But the default (and should be) is
"works basically correctly without user intervention".

[*] you can run gcc on the "right" hardware, but typically we
    build-once-run-everywhere.