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

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Dec 05, 2016 at 04:21:22PM +0100, Greg Kurz wrote:
> The current code recursively applies global properties from child up to
> parent. So, if you have:
> 
> -global virtio-pci.disable-modern=on
> -global virtio-blk-pci.disable-modern=off
> 
> Then the default value of disable-modern for a virtio-blk-pci device is on,
> which looks wrong from an OOP perspective.
> 
> This patch reverses the logic, so that a child property always prevail.
> 
> This fixes a subtle bug that got introduced in 2.7 with commit "9a4c0e220d8a
> hw/virtio-pci: fix virtio behaviour" for older (< 2.7) machine types: the
> HW_COMPAT_2_6 macro contains global virtio-pci.disable-* properties which
> would silently override global properties passed on the command line for
> virtio subtypes.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
> 
> 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.

FYI, we explicitly do *not* guarantee any kind of stability/compatibility
if you use  qemu:commandline. Users should expect it to break when they
upgrade QEMU and/or libvirt.

Regards,
Daniel
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