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

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Message-ID: <472E50C5.5020201@goop.org>
Date:	Sun, 04 Nov 2007 15:07:49 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Mikael Pettersson <mikpe@...uu.se>
CC:	hpa@...or.com, linux-kernel@...r.kernel.org, mingo@...hat.com,
	tglx@...utronix.de
Subject: Re: [PATCH] fix i486 boot failure due to stale %ds

Mikael Pettersson wrote:
> On Sun, 04 Nov 2007 11:41:58 -0800, H. Peter Anvin wrote:
>   
>> Mikael Pettersson wrote:
>>     
>>> First patch didn't build. Second patch builds and boots Ok.
>>>
>>> So this means the 486 DX4 has a buggy mov to %cr0?
>>>
>>>       
>> Apparently.
>>     
>
> 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...

    J
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