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

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Message-ID: <472E557E.3010903@goop.org>
Date:	Sun, 04 Nov 2007 15:27:58 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	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:
> 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).

Yes, it says it needs to be a far jmp or call (and if you enabled paging
in the cr0 load, you need to identity-map the branch target).  Having
successfully broken the rules for a long time so far, maybe we can get
away with still cutting corners...  but it doesn't seem particularly
worthwhile since we've been caught once.

    J
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