Re: [Qemu-devel] [PATCH for-2.8] qdev: apply global properties in reverse order

[Greg Kurz <groug@kaod.org>]

On Mon, 5 Dec 2016 17:01:28 +0100
Cornelia Huck <cornelia.huck@de.ibm.com> wrote:

> On Mon, 5 Dec 2016 16:42:00 +0100
> Cornelia Huck <cornelia.huck@de.ibm.com> wrote:
> 
> > On Mon, 05 Dec 2016 16:21:22 +0100
> > Greg Kurz <groug@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 :)