linux-kernel - Re: [PATCH] fix i486 boot failure due to stale %ds

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Message-ID: <472E5AB6.40509@zytor.com>
Date:	Sun, 04 Nov 2007 15:50:14 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Mikael Pettersson <mikpe@...uu.se>, linux-kernel@...r.kernel.org,
	mingo@...hat.com, tglx@...utronix.de,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] fix i486 boot failure due to stale %ds

Jeremy Fitzhardinge wrote:
>> Maybe not. I had a look in Intel's SDM Vol3, and the
>> section "switching to protected mode" specifies that
>> a move to %cr0 that sets PE should immediately be
>> followed by a far jmp or call.
> 
> Yes, that's what the spec says.  I queried this a few months ago, but
> hpa used his convincing voice and said that in practice it isn't
> necessary; there are no known cpus which need this, and any that do
> would cause other things to break.  But I guess now we have the
> counter-example...

Joy.  Apparently the Intel documentation is actually self-inconsistent. 
  Section 9.9.1, page 9-17 does indeed have the "far jump or call" 
injunction, whereas the sample code in section 9.10.1, page 9-27, line 
180 does a near jump!

	-hpa
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