linux-kernel - Re: [GIT PULL] x86 setup: correct booting on 486DX4

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Date:	Sun, 4 Nov 2007 15:25:06 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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



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!)

			Linus
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