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

[Greg Kurz <groug@kaod.org>]

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:
> 
> > 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 sounds reasonable...
> 
> > 
> > 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.  
> 
> ...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).
> 

Yeah, it's unfortunate I encountered the issue this late... I usually
pass properties to -device directly. :-\ Maybe we should set global
options per virtio-pci subtype as you suggested on IRC ?

Note: sorry for the late answer but I've just received all the mails...

> > 
> > Please advise.
> > 
> >  hw/core/qdev-properties.c |   11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/core/qdev-properties.c b/hw/core/qdev-properties.c
> > index 2a8276806721..1345f489d6b1 100644
> > --- a/hw/core/qdev-properties.c
> > +++ b/hw/core/qdev-properties.c
> > @@ -1119,11 +1119,20 @@ static void qdev_prop_set_globals_for_type(DeviceState *dev,
> >  void qdev_prop_set_globals(DeviceState *dev)
> >  {
> >      ObjectClass *class = object_get_class(OBJECT(dev));
> > +    GSList *class_list = NULL;
> > 
> >      do {
> > -        qdev_prop_set_globals_for_type(dev, object_class_get_name(class));
> > +        class_list = g_slist_prepend(class_list, class);
> >          class = object_class_get_parent(class);
> >      } while (class);
> > +
> > +    do {
> > +        GSList *head = class_list;
> > +
> > +        qdev_prop_set_globals_for_type(dev, object_class_get_name(head->data));
> > +        class_list = g_slist_next(head);
> > +        g_slist_free_1(head);
> > +    } while (class_list);
> >  }
> > 
> >  /* --- 64bit unsigned int 'size' type --- */
> >   
> 
> It is a bit unfortunate that we need a double loop here, but I don't
> see any good alternative.
>