Re: [musl] [RFC v3 1/1] xtensa: add port

[Max Filippov <jcmvbkbc@gmail.com>]

On Mon, May 6, 2024 at 6:37 PM Rich Felker <dalias@libc.org> wrote:
>
> On Mon, May 06, 2024 at 05:40:06PM -0700, Max Filippov wrote:
> > On Mon, May 6, 2024 at 4:58 PM Rich Felker <dalias@libc.org> wrote:
> > >
> > > On Mon, May 06, 2024 at 04:28:18PM -0700, Max Filippov wrote:
> > > > On Mon, May 6, 2024 at 3:55 PM Rich Felker <dalias@libc.org> wrote:
> > > > >
> > > > > On Mon, May 06, 2024 at 03:40:49PM -0700, Max Filippov wrote:
> > > > > > On Mon, May 6, 2024 at 3:15 PM Rich Felker <dalias@libc.org> wrote:
> > > > > > >
> > > > > > > On Mon, May 06, 2024 at 02:47:45PM -0700, Max Filippov wrote:
> > > > > > > > On Mon, May 6, 2024 at 1:57 PM Rich Felker <dalias@libc..org> wrote:
> > > > > > > > >
> > > > > > > > > On Mon, May 06, 2024 at 11:01:12AM -0700, Max Filippov wrote:
> > > > > > > > > > diff --git a/arch/xtensa/reloc.h b/arch/xtensa/reloc.h
> > > > > > > > > > new file mode 100644
> > > > > > > > > > index 000000000000..cd7a455a2d9c
> > > > > > > > > > --- /dev/null
> > > > > > > > > > +++ b/arch/xtensa/reloc.h
> > > > > > > > > > @@ -0,0 +1,32 @@
> > > > > > > > > > +#if __FDPIC__
> > > > > > > > > > +#define ABI_SUFFIX "-fdpic"
> > > > > > > > > > +#else
> > > > > > > > > > +#define ABI_SUFFIX ""
> > > > > > > > > > +#endif
> > > > > > > > > > +
> > > > > > > > > > +#define LDSO_ARCH "xtensa" ABI_SUFFIX
> > > > > > > > >
> > > > > > > > > The ldso name is still missing endianness, if it's intended that both
> > > > > > > > > be supported. It needs to completely identify the ABI whenever there
> > > > > > > > > are incompatible ABI variants.
> > > > > > > >
> > > > > > > > For each xtensa core there's only one fixed endianness and code
> > > > > > > > built for one xtensa core is not supposed to be used for any other
> > > > > > > > core, so it's not an issue, right?
> > > > > > >
> > > > > > > Yes, it is an issue. The ldsonames for ABIs must be globally unique.
> > > > > > > They are intended to be installable in a filesystem shared between
> > > > > > > multiple archs, possibly even unrelated archs executed via qemu-user
> > > > > > > or similar.
> > > > > >
> > > > > > That means an unbound number of libraries, one per xtensa core
> > > > > > configuration and the solution that comes to mind is using xtensa
> > > > > > core name as a part of ABI name. This is a bit complicated by the
> > > > > > fact that core names are not guaranteed to be globally unique, but
> > > > > > does that sound reasonable in general?
> > > > >
> > > > > Can you describe what the parameter space of core configurations is?
> > > >
> > > > The extensible set of architectural options plus the extensible core
> > > > instruction set plus variable instruction encoding.
> > > > As I said earlier the Tensilica's own approach to it is not to try to figure
> > > > out what configurations are compatible with each other but to treat each
> > > > configuration as a separate base ABI and have things like call0/windowed
> > > > be the variations of that base ABI.
> > > >
> > > > > Does it actually make mutually incompatible ABIs? If they have the
> > > > > same instruction encoding, endianness, calling convention, etc. they
> > > > > should not be incompatible, but maybe I'm missing something unique to
> > > > > how xtensa works..?
> > > >
> > > > No, most of them fall into one of the big groups of ABIs compatible with
> > > > each other, but it is usually hard to say which ones, especially with the
> > > > little information that we have as the end users. And the number of
> > > > groups grows over time and is not limited.
> > >
> > > That kind of thing doesn't need different ldso names. The name needs
> > > to identify the linkage boundary, not any ISA extensions the
> > > application or libc/ldso might be using.
> >
> > I'm not sure I understand what "linkage boundary" means. A barrier that
> > would prevent linking two pieces of code that cannot work together by
> > design?
> >
> > > As an analogy, you could
> > > build i386 musl with -march for intel/sse2 or for amd/k6/3dnow, and
> > > these would be mutually incompatible ISA extensions, but there's
> > > nothing incompatible about the ABI/linkage.
> >
> > I'm not sure how this is compatible with the
> >
> > > > > > > They are intended to be installable in a filesystem shared between
> > > > > > > multiple archs, possibly even unrelated archs executed via qemu-user
> > > > > > > or similar.
> >
> > If a library built for sse2 cannot run on k6 but may still have the
> > same name, that breaks the filesystem where it is installed, either
> > for intel or for amd, right? In that case that filesystem can only function
> > on intel or on amd. If intel and amd are analogues for two specific
> > xtensa core configurations there's no need to differentiate on the
> > endianness, because each core configuration has single fixed
> > endianness.
>
> It means you can install a libc that is compatible with either one by
> refraining from building it with extensions that preclude using it
> with both.

This part is not feasible for xtensa: there's no single base instruction
set compatible with all xtensa cores of the same endianness.

> > > Likewise, on arm you might
> > > have some chips that don't support thumb and others that don't support
> > > 32-bit arm instructions, but either way the linkage is compatible and
> > > you can call between them on any environment that supports both.
> >
> > On xtensa systems one cannot choose to build little- or big-endian code
> > for the given core like it is possible to choose whether to build FDPIC or
> > non-FDPIC code.
>
> Indeed. Little- and big-endian are incompatible ABIs via how they
> define the representation of types differently. FDPIC and non-FDPIC
> are incompatible ABIs via how the calling convention and
> representation of function pointers differ.
>
> Do these answers help clarify what linkage boundary means above?

I believe that in accordance with how Tensilica treats xtensa cores,
core configuration should be one of the linkage boundaries, along with
the FDPIC/non-FDPIC and call0/windowed. So ldso names would look
like xtensa-dc233c-fdpic.

-- 
Thanks.
-- Max