From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754680AbXKDXSf (ORCPT ); Sun, 4 Nov 2007 18:18:35 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752867AbXKDXS2 (ORCPT ); Sun, 4 Nov 2007 18:18:28 -0500 Received: from smtp2.linux-foundation.org ([207.189.120.14]:39207 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752168AbXKDXS1 (ORCPT ); Sun, 4 Nov 2007 18:18:27 -0500 Date: Sun, 4 Nov 2007 15:17:48 -0800 (PST) From: Linus Torvalds To: "H. Peter Anvin" cc: Linux Kernel Mailing List , Thomas Gleixner , Ingo Molnar , Mikael Petterson , Eric Biederman Subject: Re: [GIT PULL] x86 setup: correct booting on 486DX4 In-Reply-To: <200711042259.lA4Mxa0n025210@tazenda.hos.anvin.org> Message-ID: References: <200711042259.lA4Mxa0n025210@tazenda.hos.anvin.org> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org 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