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

[Greg Kurz <groug@kaod.org>]

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On Tue, 6 Dec 2016 09:47:08 +0000
Stefan Hajnoczi <stefanha@gmail.com> wrote:

> 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.
> > 
> > FWIW I guess it could help to have this fix in 2.8, and also probably in
> > 2.7.1.  
> 
> Hi Greg,
> I won't merge this for QEMU 2.8 because this 2.7 issue is not a 2.8
> release blocker and it's too risky (good points have been raised in this
> thread).
> 
> Please target -stable when consensus has been reached.
> 

Sure I'll just do that.

Cheers.

--
Greg

> Thanks,
> Stefan


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