On Mon, 5 Dec 2016 17:01:28 +0100 Cornelia Huck <cornelia.h...@de.ibm.com> wrote:
> On Mon, 5 Dec 2016 16:42:00 +0100 > Cornelia Huck <cornelia.h...@de.ibm.com> wrote: > > > On Mon, 05 Dec 2016 16:21:22 +0100 > > Greg Kurz <gr...@kaod.org> 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 > > > <qemu:commandline> > > > 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 :)