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Re: 32-bit ABI

To: Stanislaw Skowronek <sskowron@ET.PUT.Poznan.PL>
Subject: Re: 32-bit ABI
From: Ralf Baechle <ralf@linux-mips.org>
Date: Sat, 24 Apr 2004 09:17:52 +0200
Cc: linux-mips@linux-mips.org
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References: <Pine.LNX.4.55.0404231849480.14494@jurand.ds.pg.gda.pl> <Pine.GSO.4.10.10404240825540.10762-100000@helios.et.put.poznan.pl>
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On Sat, Apr 24, 2004 at 08:28:09AM +0200, Stanislaw Skowronek wrote:

> why do we attempt to compile the kernel with 32-bit GAS abi and 64-bit GCC
> abi? Is it because the module loader is broken and supports only 32-bit
> ELFs? Then what about machines which load their kernels at weird 64-bit
> addresses, like 0xa800000020004000 (Octane)?

The whole thing was born as a dirty hack back in '99 or so to avoid the
totally broken and imcomplete implementation of 64-bit MIPS ELF in binutils.
By using these options we run the kernel in CKSEG0 which means 32-bit ELF
will suffice.  A nice side effect if a reduction of kernel size - this in
the end made the code model which was born as a hack the way of choice.

So there is no relation at all to modules.  You btw. can load 64-bit ELF
modules into a kernel which was built using above trick as 32-bit ELF.
That's necessary because modules are currently allocated through vmalloc
which allocates space in XKSEG.

A code model which I'm considering as alternative is -G optimization but
that will require a good bit of work; we would have to free up $28 which
right now we're already using for the current pointer.  A bunch of
declaration would simply need to be fixed.  Module loaders would need
support for R_MIPS_GPREL16 and we'd need a solution for the problem of
the kernel being too large, therefore overflowing the small data section.
Th latter is probably going to require binutils hacking, so non-trivial.

> I have changed it to 64-bit abi in my Octane kernel, because it won't even
> compile otherwise. I've got gcc 3.3.2, gas 2.14.

Octane has no memory at all in CKSEG0?

>   Paranoid: one who is truly in touch with reality.

The fact that you're paranoid doesn't meant they're not out to get you.

  Ralf

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