linux-kernel - Re: x86 setup rewrite tree ready for flamage^W review

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Message-ID: <m1iraxr95g.fsf@ebiederm.dsl.xmission.com>
Date:	Sat, 12 May 2007 16:04:27 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Alexander van Heukelum <heukelum@...lshack.com>,
	"Antonino A. Daplas" <adaplas@...il.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...l.org>,
	Matt Domsch <Matt_Domsch@...l.com>,
	Vivek Goyal <vgoyal@...ibm.com>,
	James Bottomley <James.Bottomley@...senPar>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x86 setup rewrite tree ready for flamage^W review

"H. Peter Anvin" <hpa@...or.com> writes:

> Eric W. Biederman wrote:
>> 
>> Even on 386 and 486 class cpus?
>> 
>
> Yes, even on 386 and 486 class CPUs.  I have personally tested this on
> machines as old as the original "double sigma" 386-16.

Ok.  If you have tested on a wide variety of machines then I won't
worry about it.

I guess if a cr0 write has always been synchronizing things should be
a safe practice.  The practical danger is if you write to cr0 and the
pipeline is not flushed and the segment loads might execute as 16bit
mode segment register loads.  I think this is more of a 486 or 586/Pentium
danger actually then a 386 one.

Hmm. I'm not certain about enabling protected mode but enabling paging
at least does appear to be documented to require a jump, before the P6
core.  Which may be why it is recommended for initializing protected
mode.

Eric
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