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

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Date:	Thu, 15 Jan 2009 15:37:12 +0100
From:	"Bastien ROUCARIES" <roucaries.bastien@...il.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	"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, Jan 15, 2009 at 3:20 PM, Jan-Benedict Glaw <jbglaw@...-owl.de> wrote:
> 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?)

here http://thread.gmane.org/gmane.linux.kernel/205839, but should be
securized (use get_user() )

Regards

> 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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>
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