[musl] Re: ppc64le and 32-bit LE userland compatibility

[Rich Felker]

On Mon, Jun 01, 2020 at 09:28:25PM +0000, Joseph Myers wrote:
> On Fri, 29 May 2020, Will Springer via Binutils wrote:
> 
> > Hey all, a couple of us over in #talos-workstation on freenode have been
> > working on an effort to bring up a Linux PowerPC userland that runs in 32-bit
> > little-endian mode, aka ppcle. As far as we can tell, no ABI has ever been
> > designated for this (unless you count the patchset from a decade ago [1]), so
> > it's pretty much uncharted territory as far as Linux is concerned. We want to
> > sync up with libc and the relevant kernel folks to establish the best path
> > forward.
> 
> As a general comment on the glibc side of things, if this is considered 
> like a new port, and it probably is, the same principles that apply to new 
> ports apply here.
> 
> There's a general discussion at 
> <https://sourceware.org/glibc/wiki/NewPorts>, although much of that is 
> only applicable when adding new CPU architecture support.  More specific 
> points include that new 32-bit ports should default to 64-bit time and 
> file offsets from the start, with no support for 32-bit time or offsets 
> (meaning that if you want to use this with some kind of library call 
> translation, the library call translation will need to deal with 
> corresponding type size conversions).  And a new port should not be added 
> that uses the IBM long double format.  You can use IEEE binary128 long 
> double, possibly with an ABI similar to that used on powerpc64le, or can 
> use long double = double, but should not support IBM long double, and 
> preferably should only have one long double format rather than using the 
> glibc support for building with different options resulting in functions 
> for different long double formats being called.

Thanks, these are great points, and the same applies for musl I think.
We always have 64-bit off_t anyway, but new ports should have 64-bit
time_t to begin with rather than defining _REDIR_TIME64.

It's a little bit complicated by the fact that powerpcle would be a
"subarch" for powerpc, and we don't yet have any like that where the
subarchs' time64 statuses differ, but it doesn't look like that should
be hard to do. The arch-specific alltypes.h.in already has access to
endianness knowledge. src/ldso/powerpc/dlsym_time64.S would also need
added preprocessor conditionals.

Exemption from this would be open to discussion if there are existing
non-upstream users of powerpcle musl that otherwise complies with ABI
policy except for time64, but I'm not aware of any that aren't
experimental.

Rich