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Re: [PATCH] mm/pg-r4k.c: Dump the generated code

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: [PATCH] mm/pg-r4k.c: Dump the generated code
From: Franck Bui-Huu <vagabon.xyz@gmail.com>
Date: Thu, 04 Oct 2007 17:01:32 +0200
Cc: Thiemo Seufer <ths@networkno.de>, "Maciej W. Rozycki" <macro@linux-mips.org>, linux-mips@linux-mips.org
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Ralf Baechle wrote:
> On Thu, Oct 04, 2007 at 09:33:08AM +0200, Franck Bui-Huu wrote:
> 
>> Not really, I would say it's just an idea to remove tlbex.c from the
>> kernel code and to make it a tool called during compile time to
>> generate a handler skeleton which would be finalized by the kernel.
> 
> IRIX was assembling its TLB exception handler from a few such skeletons
> or rather a few fractions.  That works reasonably well as long as there are
> not too many variants - but Linux supports about anything on earth.
> Another disadvantage of the IRIX approach was that the fragments are
> written in assembler but the tacking together happens in C code so the
> code is split in a somewhat unnatural way over a few files.
> 

That's what I was thinking too. It may require a lot of (ugly ?)
tricks to link the whole thing together. And if the idea was
previously used and showed it was inferior than what we have now, it's
just a bad idea.

It's just a bit sad to see my TLB handler generated at each boot and
to embed the whole tlbex generator inside the kernel which is quite
big:

   $ mipsel-linux-size arch/mips/mm/tlbex.o
      text    data     bss     dec     hex filename
     10116    3904    1568   15588    3ce4 arch/mips/mm/tlbex.o

specially if my cpu doesn't have any bugs.

Maybe having, 2 default implementations in tlbex-r3k.S, tlbex-r4k.S
for good cpus (the ones which needn't any fixups at all) and otherwise
the tlbex.c is used. And with luck the majority of the cpus are
good...

OK, probably a bad idea (again) ...

Thanks
                Franck

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