linux-kernel - Re: [patch 0/8] Immediate Values

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Date:	Sun, 12 Aug 2007 17:14:16 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 0/8] Immediate Values

* Arjan van de Ven (arjan@...radead.org) wrote:
> > > 
> > > I have a concern; you seem to be patching potentially "live" code....
> > > 
> > > there are basically two options
> > > 1) you run the risk of triple faulting (patching an instruction while
> > > some other core/cpu may be decoding it may cause a triple fault)
> > > 2) you do an IPI to all other cpus and prevent them from executing any
> > > code except a small loop during the patching... this is expensive.
> > > 
> > > To be honest, neither sound very attractive to me ;(
> > > 
> > 
> > Yup, the concern is appropriate. That's why I dealt with it in the
> > "Immediate Values - i386 Optimization" patch. (I guess your concern
> > is specific to i386, x86_64 and ia64).
> 
> it's specific to all smp architectures I suppose

I would wait to see the references before doing such presumptions..
AFAIK, powerpc does not suffer from such problems. I have also seen
nothing of this kind about AMD processors, so it seems to be Intel
specific.

> > 
> > I have currently only implemented the i386 optimization, but x86_64 and
> > ia64 should be similar.
> > 
> > The triple fault in question is discussed in Intel's errata under the
> > title "Centrino Duo Processor Technology Specification Update, AH33.
> > Unsynchronized Cross-Modifying Code Operations Can Cause Unexpected
> > Instruction Execution Results." (if you refer to something else, please
> > tell me).
> 
> Core2DUO is only one generation of Intel processors; however I know that
> several other (if not all of them) have issues with this kind of thing
> in ring 0; all subtly different ;(
> 

Yes, this errata can be found starting at the Pentium III (as explained
in my patch). Can you elaborate a little more on the differences ? From
what I have seem, the basic problem is always the same from PIII to
core2 Duo: instruction prefetch.


> All other alternatives are ok right now (they all run in UP mode),
> but... I'm just very nervous about the amount of CPU errata in this kind
> of scenario. And I'm not just talking Intel, I wouldn't be surprised if
> the other x86 CPU vendors also have issues with this; I don't know how
> well they specify their errata though..
> 
> 

You seem to be pointing out to a lot of erratas, when there is, from
what I reckon, only one (basically the same) from PIII to Core 2 Duo,
which is Intel specific. I have looked around and ia64 also seems to be
affected by this. Can you point us out to other documentation sources
please ?

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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