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Re: 2GB userspace limitation in ABI N32

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: 2GB userspace limitation in ABI N32
From: Ronny Meeus <ronny.meeus@gmail.com>
Date: Wed, 10 Oct 2012 17:12:16 +0200
Cc: linux-mips@linux-mips.org
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Do you have any clue (rough) on the amount of effort this change would cost?

About the limited gain we can discuss: if you have a large application
that has been created assuming 32bit and it needs to be ported to a
64bit architecture, I think the effort can be huge and the risk for
forgetting things is high. It will be very hard to check whether the
system behaves well under all conditions.

---
Ronny

On Wed, Oct 10, 2012 at 10:07 AM, Ralf Baechle <ralf@linux-mips.org> wrote:
> On Wed, Oct 10, 2012 at 08:32:47AM +0200, Ronny Meeus wrote:
>
>> I have a legacy application that we want to port to a MIPS (Cavium)
>> architecture from a PPC based one.
>> The board has 4GB memory of which we actually need almost 3GB in
>> application space. On the PPC this is no issue since the split
>> user/kernel is 3GB/1GB.
>> We have to use the N32 ABI Initial tests on MIPS showed me the
>> user-space limit of 2GB.
>> We do not want to port the application to a 64bit
>>
>> Now the question is: are there any workarounds, tricks existing to get
>> around this limitation?
>> I found some mailthreads on this subject (n32-big ABI -
>> http://gcc.gnu.org/ml/gcc/2011-02/msg00278.html,
>> http://elinux.org/images/1/1f/New-tricks-mips-linux.pdf) but is looks
>> like this is not accepted by the community. Is there any process
>> planned or made in this area?
>
> I think limited time and gain killed the propoosed ABI rather than
> theoretical issues raised.  Other architectures such as i386 - well,
> IIRC any 32-bit ABI with more than 2GB userspace and a signed
> ptrdiff_t - are suffering from them as well.
>
> Also there's limited gain and even more limited time to implement things ...
>
>   Ralf

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