From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262665AbULPOUz (ORCPT ); Thu, 16 Dec 2004 09:20:55 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262670AbULPOUy (ORCPT ); Thu, 16 Dec 2004 09:20:54 -0500 Received: from clock-tower.bc.nu ([81.2.110.250]:8578 "EHLO localhost.localdomain") by vger.kernel.org with ESMTP id S262665AbULPOUG (ORCPT ); Thu, 16 Dec 2004 09:20:06 -0500 Subject: Re: arch/xen is a bad idea From: Alan Cox To: Andi Kleen Cc: Ian Pratt , Rik van Riel , Linux Kernel Mailing List , akpm@osdl.org, Steven.Hand@cl.cam.ac.uk, Christian.Limpach@cl.cam.ac.uk, Keir.Fraser@cl.cam.ac.uk In-Reply-To: <20041216140954.GA29761@wotan.suse.de> References: <20041215044927.GF27225@wotan.suse.de> <1103155782.3585.29.camel@localhost.localdomain> <20041216040136.GA30555@wotan.suse.de> <1103201656.3804.7.camel@localhost.localdomain> <20041216140954.GA29761@wotan.suse.de> Content-Type: text/plain Content-Transfer-Encoding: 7bit Message-Id: <1103203145.3804.13.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Ximian Evolution 1.4.6 (1.4.6-2) Date: Thu, 16 Dec 2004 13:19:06 +0000 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Iau, 2004-12-16 at 14:09, Andi Kleen wrote: > Also e.g. for non performance critical > things like changing MTRRs or debug registers it would be IMHO much > cleaner to just emulate the instructions (the ISA is very well > defined) and not change the kernel here. From a look at Ian's list > the majority of the changes needed for Xen actually fall into > this category. There are so many problems in snooping and decoding instructions it isn't funny. Aside from the mmap pci buffer half way through instruction that will emulate type stuff there are a lot of awkward issues if you want to emulate multiple mtrr sets (you need PAT). Xen has tried the decode/patch stuff earlier - see the early NPTL handling and it was neither pretty nor reliable.