linux-kernel - Re: Is 386 processor still supported?

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Message-ID: <20090115142047.GV11087@lug-owl.de>
Date:	Thu, 15 Jan 2009 15:20:47 +0100
From:	Jan-Benedict Glaw <jbglaw@...-owl.de>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Jiri Kosina <jkosina@...e.cz>, Adam Osuchowski <adwol@...k.pl>,
	linux-kernel@...r.kernel.org, Nick Piggin <npiggin@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: Is 386 processor still supported?

On Thu, 2009-01-15 14:15:56 +0000, Maciej W. Rozycki <macro@...ux-mips.org> wrote:
> On Thu, 15 Jan 2009, Alan Cox wrote:
> > >  UP emulation of CMPXCHG and XADD for userland should be rather trivial, 
> > > so why not include it like with LL/SC for MIPS?
> > 
> > Why not just ship an additional libc with the right options ?
> 
>  Does not work for MIPS as glibc has no equivalent code for pre-LL/SC CPUs 
> and LL/SC is always used.  For the i386 the situation seems worse yet as 
> for pre-i486 CPUs a generic C implementation of compare-and-exchange is 
> used guaranteeing silent thread unsafety. :(
> 
>  IMO, a kernel emulation of CMPXCHG and XADD (both are used by 
> sysdeps/i386/i486/bits/atomic.h in glibc) with an optional LOCK prefix, 
> guaranteeing UP atomicity would be a cheap way to provide long-term i386 
> userland support with little burden for both Linux and respective user 
> software maintainers.  Certainly it adds some bloat to the kernel, but I 
> think it is not an option that should be outright dismissed without 
> consideration.

I just searched for the old patch, but couldn't find it ad hoc. (But
it must be somewhere, at least in the archives, I guess?)

The kernel emulator has the benefit of no overhead when not switched
on, and low-to-no overhead when not being used (i386 capable kernel on
i486 hardware).

Heck, I'd dig out my two test systems and give them a try with current
Debian unstable. Should be fun with four to eight megabytes of RAM.

MfG, JBG

-- 
      Jan-Benedict Glaw      jbglaw@...-owl.de              +49-172-7608481
Signature of: 23:53 <@jbglaw> So, ich kletter' jetzt mal ins Bett.
the second  : 23:57 <@jever2> .oO( kletter ..., hat er noch Gitter vorm Bett, wie früher meine Kinder?)
              00:00 <@jbglaw> jever2: *patsch*
              00:01 <@jever2> *aua*, wofür, Gedanken sind frei!
              00:02 <@jbglaw> Nee, freie Gedanken, die sind seit 1984 doch aus!
              00:03 <@jever2> 1984? ich bin erst seit 1985 verheiratet!

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