[Bug 204125] FTBFS on ppc64 big endian and gcc9 because of -mcall-aixdesc and missing __linux__

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https://bugzilla.kernel.org/show_bug.cgi?id=204125

Segher Boessenkool (segher@kernel.crashing.org) changed:

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--- Comment #1 from Segher Boessenkool (segher@kernel.crashing.org) ---
(In reply to Daniel Kolesa from comment #0)
> On ppc64 big endian, the kernel builds with `-mcall-aixdesc` which since gcc
> 9.x removes `__linux__` from the list of macros being defined.

This is a bug.  Please report at https://gcc.gnu.org/bugzilla .

> This behavior
> is supposed to be more correct (as it's in this case nothing but a hack, the
> flag should apparently only be used when building for AIX)

What makes you think that?

OTOH, why does the kernel use that option?

> but sadly it
> breaks build since several things within the tree rely on `__linux__` being
> defined and `#ifdef` some of their code based on said macro.

Those are bugs as well, then.

> Just removing `-mcall-aixdesc` (and using just `-mabi=elfv1`) is however not
> enough, as that instead causes countless undefined references to just about
> every symbol when linking `vmlinux`. It would seem that `-mcall-aixdesc`
> changes the way symbols are declared in a way that is not expected.

> Little endian is not affected because that one uses `-mabi=elfv2`
> exclusively.

Of course, that is the only defined ABI for powerpc64le after all.

> Also, is there any reason not to allow an ELFv2 kernel to be built on big
> endian?

Building it _on_ BE works just fine, of course.  But you mean building a BE
kernel using the ELFv2 ABI.  This is not supported; it would require writing
other versions for various low-level things.

ELFv2 is not supported in BE userland, either, btw.

> There are some supposed performance benefits, and ELFv2 itself
> supports either endianness. The current kbuild logic pretty much forces
> ELFv1 for big endian and ELFv2 for little endian.

ELFv2 has a few little benefits; it is newer, there were lessons learnt.  It
would be surprising if it has better than trivial advantages for the BE kernel
though.  But feel free to try, of course :-)

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