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

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Date:	Thu, 15 Jan 2009 14:15:56 +0000 (GMT)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Jan-Benedict Glaw <jbglaw@...-owl.de>, 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, 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.

  Maciej
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