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Date: Mon, 6 May 2024 19:59:01 -0400
From: Rich Felker <dalias@...c.org>
To: Max Filippov <jcmvbkbc@...il.com>
Cc: musl@...ts.openwall.com
Subject: Re: [RFC v3 1/1] xtensa: add port
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@...c.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@...c.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@...c.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. 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. 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.
It sounds like the things that need ldso name distinctions are
endianness, fdpic, and possibly the call0/windowed convention -- I'm
not clear on what that means so I'm not sure. But we should probably
review this in some detail to make sure nothing else is overlooked.
Rich
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