From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758653Ab0GHWAu (ORCPT ); Thu, 8 Jul 2010 18:00:50 -0400 Received: from mx1.redhat.com ([209.132.183.28]:27141 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755925Ab0GHWAt (ORCPT ); Thu, 8 Jul 2010 18:00:49 -0400 Message-ID: <4C364A3F.8020900@redhat.com> Date: Thu, 08 Jul 2010 17:59:27 -0400 From: Don Dutile Reply-To: ddutile@redhat.com User-Agent: Thunderbird 2.0.0.18 (X11/20081113) MIME-Version: 1.0 To: Ian Campbell CC: Stefano Stabellini , "jeremy@goop.org" , "xen-devel@lists.xensource.com" , "stefano@stabellini.net" , "linux-kernel@vger.kernel.org" , "sheng@linux.intel.com" Subject: Re: [Xen-devel] Re: [PATCH 12/13] Unplug emulated disks and nics References: <1277136847-13266-12-git-send-email-stefano@stabellini.net> <4C2CEF56.4050008@redhat.com> <4C34DD1B.3010601@redhat.com> <4C362DC3.7000101@redhat.com> <1278624583.12109.15.camel@localhost.localdomain> In-Reply-To: <1278624583.12109.15.camel@localhost.localdomain> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Ian Campbell wrote: > On Thu, 2010-07-08 at 20:57 +0100, Don Dutile wrote: >> I guess what I'm wondering is why not set xen_emul_unplug to ignore by >> default (static int xen_emul_unplug=XEN_UNPLUG_IGNORE), which handles >> the case I mentioned (just take existing guest config file as is, no edits, >> pre-pv-hvm added to guest kernel), and if person edits config file to >> change boot device to xvda, they would then edit the config to add >> -x xen_emul_unplug=[all|ide-disks|...] as well. > > Can you guarantee that nobody is running an HVM guest today with a > configuration file that specifies xvda (I believe it would work)? In > other words can you be sure that defaulting to XEN_UNPLUG_IGNORE is > _always_ going to be safe? Not just on RHEL hosts and with > configurations generated by the RH tools or according to the RH docs but > on any host with any (possibly hand-crafted) configuration? > No, you have a valid point. We have pv-on-hvm support for rhel3->rhel5 HVM guests, and they support xvda on boot devices (once initrd is rebuilt), so it's possible to have that config (spec'd) as well, and someone to copy & edit it for use on a more current disk image w/relatively current kernel. But I'm considering 2.6.32+ HVM guests that didn't have xvda spec'd in the boot path ever, and are upgraded to a post-2.6.32 kernel that has pv-on-hvm added to it. > Any guest which uses xvda in its configuration file today will be using > emulated devices but I think that with Stefano's patch and your proposed > change in default on a Xen system without support for unplug will start > using PV devices without unplugging the emulated ones first. > Well, Stefano requires the admin to add unplug switch to kernel cmd line, so I don't see the harm in defaulting to unplug... > I don't think there is any way for a guest running on a platform which > does not support the unplug protocol to know automatically if it is safe > to use the PV devices or not, therefore we have to err on the side of > caution and ask users with such systems who know that their > configuration is safe to explicitly request PV devices by using the > command line option. Doing anything else is taking risks with people's > data. > > Ian. > > Either way, the user/admin has to add cmdline to unplug to be safe. I don't see how defaulting to UNPLUG_IGNORE changes that requirement. ... or am I missing a case? -- ah, if IGNORE isn't spec'd, pvhvm just won't be configured in, and blkfront wont run, and cant have blkfront & ide accessing the same device.... is that the case I'm missing ? - Don