From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936764AbXGSBWB (ORCPT ); Wed, 18 Jul 2007 21:22:01 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1763455AbXGSBVt (ORCPT ); Wed, 18 Jul 2007 21:21:49 -0400 Received: from terminus.zytor.com ([198.137.202.10]:46859 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762640AbXGSBVt (ORCPT ); Wed, 18 Jul 2007 21:21:49 -0400 Message-ID: <469EBC63.9020803@zytor.com> Date: Wed, 18 Jul 2007 18:20:35 -0700 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.0 (X11/20070419) MIME-Version: 1.0 To: Andi Kleen CC: Avi Kivity , Linus Torvalds , kvm-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org, caglar@pardus.org.tr Subject: Re: [PATCH] i386: Decouple PAE from CONFIG_CMPXCHG64 References: <11847764879-git-send-email-avi@qumranet.com> <20070718181829.GA3848@one.firstfloor.org> In-Reply-To: <20070718181829.GA3848@one.firstfloor.org> X-Enigmail-Version: 0.95.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Andi Kleen wrote: >> >> No processors that support KVM exist that also do not support CMPXCHG64, >> so no additional check is necessary. This setup allows for a single kernel >> that will boot on i486 and also support KVM if available. > > The CONFIG should only control the early CPUID checks, which are needed for PAE > kernels. > > But for something late like KVM the correct fix is really to drop > the ifdefs around the macro and check boot_cpu_has() yourself. > He's checking for the appropriate functionality at runtime. What this does is makes it impossible to select KVM when you have already selected a CPU that doesn't support CX8. One can, of course, question the utility, since the CPU set that supports KVM all support CX8, but not vice versa... -hpa