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Date: Sun, 04 Nov 2007 15:36:37 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
       Mikael Petterson <mikpe@it.uu.se>,
       Eric Biederman <ebiederm@xmission.com>
Subject: Re: [GIT PULL] x86 setup: correct booting on 486DX4
References: <200711042259.lA4Mxa0n025210@tazenda.hos.anvin.org> <alpine.LFD.0.999.0711041512530.15101@woody.linux-foundation.org> <alpine.LFD.0.999.0711041522300.15101@woody.linux-foundation.org>
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Linus Torvalds wrote:
> 
> On Sun, 4 Nov 2007, Linus Torvalds wrote:
>> I'm not entirely sure that it needs to be a long-jump, btw. I think any
>> regular branch is sufficient. You obviously *do* need to make the long 
>> jump later (to reload %cs in protected mode), but I'm not sure it's needed 
>> in that place. I forget the exact rules (but they definitely were 
>> documented).
> 
> Hmm. The original Linux code did
> 
> 	movw    $1, %ax
> 	lmsw    %ax
> 	jmp     flush_instr
>     flush_instr:
> 
> and I think that was straigh out of the documentation. So yeah, I think 
> that's the right fix - not a longjmp (which in itself is dangerous: it 
> potentially behaves *differently* on different CPU's, since some CPU's may 
> do the long jump with pre-protected-mode semantics, while others will do 
> it with protected mode already in effect!)
> 

Just looked it up; it was a bit hard to find (it is Intel vol 3 page 
9-27, at least in the version I have), but you're right -- the 
documentation only demands a short jump here, not a long jmp (which 
actually makes sense given what I remembered that a long jump should be 
deferrable here.)  So yes, that is definitely the right fix and avoids 
the ugly mixing of code.

I'll update the patch.

	-hpa