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Date: Mon, 5 Dec 2016 17:01:28 +0100
From: Cornelia Huck <cornelia.huck@de.ibm.com>
In-Reply-To: <20161205164200.49bec0f6.cornelia.huck@de.ibm.com>
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Message-Id: <20161205170128.533850e5.cornelia.huck@de.ibm.com>
Subject: Re: [Qemu-devel] [PATCH for-2.8] qdev: apply global properties in
 reverse order
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To: Greg Kurz <groug@kaod.org>
Cc: qemu-devel@nongnu.org, Peter Maydell <peter.maydell@linaro.org>, Marcel Apfelbaum <marcel.apfelbaum@gmail.com>, "Michael S. Tsirkin" <mst@redhat.com>, qemu-stable@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>

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.

> > 
> > 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...