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Message-ID: <472E5785.4020004@zytor.com>
Date: Sun, 04 Nov 2007 15:36:37 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Mikael Petterson <mikpe@...uu.se>,
Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [GIT PULL] x86 setup: correct booting on 486DX4
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
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