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

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Message-ID: <alpine.LFD.0.999.0711041512530.15101@woody.linux-foundation.org>
Date:	Sun, 4 Nov 2007 15:17:48 -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, H. Peter Anvin wrote:
>     
>     Apparently, the 486DX4 does not correctly serialize a mov to %cr0, so
>     we really do need the far jump immediately afterwards.

Hmm. I'm not sure I agree with the commit message.

This is documented behaviour on i386 and i486: instruction decoding is 
decoupled from execution, so things that change processor mode have to do 
a jump to make sure that %cr0 changes take effect.

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).

		Linus
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