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Re: [PATCH] qemu does not have dcache aliases

To: Thiemo Seufer <ths@networkno.de>
Subject: Re: [PATCH] qemu does not have dcache aliases
From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Mon, 21 Aug 2006 17:43:29 +0200 (CEST)
Cc: Ralf Baechle <ralf@linux-mips.org>, "Maciej W. Rozycki" <macro@linux-mips.org>, Atsushi Nemoto <anemo@mba.ocn.ne.jp>, Linux/MIPS Development <linux-mips@linux-mips.org>
In-reply-to: <20060821153616.GC20395@networkno.de>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <20060820.003338.25478178.anemo@mba.ocn.ne.jp> <Pine.LNX.4.64N.0608211340120.17504@blysk.ds.pg.gda.pl> <20060821.225910.108307053.anemo@mba.ocn.ne.jp> <Pine.LNX.4.64N.0608211612090.17504@blysk.ds.pg.gda.pl> <20060821152657.GB19600@linux-mips.org> <20060821153616.GC20395@networkno.de>
Sender: linux-mips-bounce@linux-mips.org
On Mon, 21 Aug 2006, Thiemo Seufer wrote:
> Ralf Baechle wrote:
> > On Mon, Aug 21, 2006 at 04:16:21PM +0100, Maciej W. Rozycki wrote:
> > 
> > > > Well, the QEMU cpu has 2-way 2kB dcache... does not have aliasing
> > > > anyway. :-)
> > > 
> > >  I don't think emulating a bigger cache so that we can add aliases should 
> > > be *that* difficult.  Adding aliases themselves might be a bit trickier, 
> > > but the gain would certainly justify the hassle, wouldn't it?
> > 
> > The cache lookup for every access would have serious impact on performance.
> > The question is, is it worth the price?  Would such patches be acceptable
> > by the upstream qemu maintainers?
> 
> Likely not, given that performance is the prime criterion there.

It depends. If you just want to run MIPS Linux, yes.
If you want to debug the kernel on a MIPS core with cache aliasing[*], no.

[*] Replace with whatever slow-to-emulate and hard-to-debug feature.

Gr{oetje,eeting}s,

                                                Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                                            -- Linus Torvalds

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