From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39206) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cEB6d-00030w-CA for qemu-devel@nongnu.org; Tue, 06 Dec 2016 03:28:08 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cEB6Z-0006va-F3 for qemu-devel@nongnu.org; Tue, 06 Dec 2016 03:28:07 -0500 Received: from 7.mo4.mail-out.ovh.net ([178.33.253.54]:47182) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cEB6Z-0006rt-7y for qemu-devel@nongnu.org; Tue, 06 Dec 2016 03:28:03 -0500 Received: from player762.ha.ovh.net (b7.ovh.net [213.186.33.57]) by mo4.mail-out.ovh.net (Postfix) with ESMTP id 4E34E217F5 for ; Tue, 6 Dec 2016 09:28:00 +0100 (CET) Date: Tue, 6 Dec 2016 09:27:49 +0100 From: Greg Kurz Message-ID: <20161206092749.01a55bca@bahia> In-Reply-To: <20161205170128.533850e5.cornelia.huck@de.ibm.com> References: <148095126363.31351.4484514300033863622.stgit@bahia> <20161205164200.49bec0f6.cornelia.huck@de.ibm.com> <20161205170128.533850e5.cornelia.huck@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH for-2.8] qdev: apply global properties in reverse order List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Cornelia Huck , qemu-devel@nongnu.org Cc: Peter Maydell , Marcel Apfelbaum , "Michael S. Tsirkin" , qemu-stable@nongnu.org, Stefan Hajnoczi , Paolo Bonzini On Mon, 5 Dec 2016 17:01:28 +0100 Cornelia Huck wrote: > On Mon, 5 Dec 2016 16:42:00 +0100 > Cornelia Huck wrote: > > > On Mon, 05 Dec 2016 16:21:22 +0100 > > Greg Kurz wrote: > > > > AFAIK, libvirt's XML doesn't know about modern/legacy modes for virtio > > > devices. Early adopters of virtio 1.0 had to rely on the > > > tag to pass global properties to QEMU. This patch ensures that XML files > > > used with older machine types remain valid with newer versions of QEMU. > > I recall some libvirt patches floating around for this legacy/modern > stuff, but I don't know their status. > libvirt does some probing of disable-legacy but I could find nothing that allows a user to explicitly choose between legacy or modern. > > > > > > FWIW I guess it could help to have this fix in 2.8, and also probably in > > > 2.7.1. > > > > ...but I'm a bit worried about doing that change this late in the > > cycle, as we may introduce subtle changes for other configurations. At > > the very least, we should look over the existing backwards compat > > properties (I'll look at those I'm familiar with). > > The s390x properties seem safe. > > For virtio-pci, the ability to override extra state might become > problematic for modern devices. Although manually setting this property > is probably a patholotical case... > I agree that migrating a 2.6 machine type with disable-modern=off being set through -global for a given subtype, from QEMU 2.6 to QEMU 2.8 (or 2.7) is probably a corner case :)