From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1M9fWB-0001Wl-Dp for qemu-devel@nongnu.org; Thu, 28 May 2009 09:19:35 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1M9fW7-0001WR-Rw for qemu-devel@nongnu.org; Thu, 28 May 2009 09:19:35 -0400 Received: from [199.232.76.173] (port=35152 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1M9fW7-0001WO-Pe for qemu-devel@nongnu.org; Thu, 28 May 2009 09:19:31 -0400 Received: from e2.ny.us.ibm.com ([32.97.182.142]:60907) by monty-python.gnu.org with esmtps (TLS-1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1M9fW7-0001lu-CD for qemu-devel@nongnu.org; Thu, 28 May 2009 09:19:31 -0400 Received: from d01relay04.pok.ibm.com (d01relay04.pok.ibm.com [9.56.227.236]) by e2.ny.us.ibm.com (8.13.1/8.13.1) with ESMTP id n4SDFG8Q031031 for ; Thu, 28 May 2009 09:15:16 -0400 Received: from d01av03.pok.ibm.com (d01av03.pok.ibm.com [9.56.224.217]) by d01relay04.pok.ibm.com (8.13.8/8.13.8/NCO v9.2) with ESMTP id n4SDJUkV255008 for ; Thu, 28 May 2009 09:19:30 -0400 Received: from d01av03.pok.ibm.com (loopback [127.0.0.1]) by d01av03.pok.ibm.com (8.12.11.20060308/8.13.3) with ESMTP id n4SDJUSe021073 for ; Thu, 28 May 2009 09:19:30 -0400 Message-ID: <4A1E8F5D.1090408@us.ibm.com> Date: Thu, 28 May 2009 08:19:25 -0500 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] [PATCH] Change virtio-console to PCI_CLASS_SERIAL_OTHER References: <1243012478.29542.18.camel@blaa> <4A1D4C57.6010109@us.ibm.com> <1243446153.4852.9.camel@blaa> <200905281353.50463.paul@codesourcery.com> <20090528130419.GC24488@redhat.com> In-Reply-To: <20090528130419.GC24488@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Daniel P. Berrange" Cc: Mark McLoughlin , Dor Laor , ajax@redhat.com, Paul Brook , qemu-devel@nongnu.org Daniel P. Berrange wrote: > On Thu, May 28, 2009 at 01:53:49PM +0100, Paul Brook wrote: > >> On Wednesday 27 May 2009, Mark McLoughlin wrote: >> >>> On Wed, 2009-05-27 at 09:21 -0500, Anthony Liguori wrote: >>> >>>> We need a mechanism to toggle this for both this and virtio-blk. The >>>> reason a toggle is needed is so that 0.11 can create the same device >>>> model as 0.10. >>>> >>> Okay, so the scenario is: >>> >>> - 0.10 guest running on source machine >>> >>> - migrate to dest machine running 0.11 >>> >>> - the device model cannot change or the guest OS will get confused >>> >> IMHO think the only sane response is "don't do that". Trying to support >> migration between different qemu versions just isn't worth the pain. >> > > I think that further more, we shouldn't make a change like altering > the PCI device class in the stable branch, only in the unstable > branch. If we restrict the stable branch to bug fixes, then it ought > to be (more?) practical to support save under 0.10 and restore under > 0.11, without needing to support the hard problem of save under 0.10 > and restore under 1.00 (or whatever next major release branch is) > This is not a save/restore issue. PCI config will be saved in the image so everything is fine until you shut a VM down. The problem is that the device model a guest sees is an ABI. If we change the ABI between versions, the guest make break. What I'm proposing is that while we can change the default guest ABI between versions, we must provide a way to recreate older forms of the guest ABI. A good example of guests breaking is Windows activation failing. It has happened more than once that changes in QEMU cause Windows guests to force reactivation. In more extreme instances, a guest may break that used to work before. Regards, Anthony Liguori > Regards, > Daniel >