Re: [Qemu-devel] [PATCH] qdev: DEVICE_DELETED event

[Michael S. Tsirkin]

On Wed, Mar 06, 2013 at 02:57:22PM +0100, Andreas Färber wrote:
> Am 06.03.2013 14:00, schrieb Michael S. Tsirkin:
> > libvirt has a long-standing bug: when removing the device,
> > it can request removal but does not know when does the
> > removal complete. Add an event so we can fix this in a robust way.
> > 
> > Signed-off-by: Michael S. Tsirkin <address@hidden>
> 
> Sounds like a good idea to me. :)
> 
> [...]
> > diff --git a/hw/qdev.c b/hw/qdev.c
> > index 689cd54..f30d251 100644
> > --- a/hw/qdev.c
> > +++ b/hw/qdev.c
> > @@ -29,6 +29,7 @@
> >  #include "sysemu/sysemu.h"
> >  #include "qapi/error.h"
> >  #include "qapi/visitor.h"
> > +#include "qapi/qmp/qjson.h"
> >  
> >  int qdev_hotplug = 0;
> >  static bool qdev_hot_added = false;
> > @@ -267,6 +268,11 @@ void qdev_init_nofail(DeviceState *dev)
> >  /* Unlink device from bus and free the structure.  */
> >  void qdev_free(DeviceState *dev)
> >  {
> > +    if (dev->id) {
> > +        QObject *data = qobject_from_jsonf("{ 'device': %s }", dev->id);
> > +        monitor_protocol_event(QEVENT_DEVICE_DELETED, data);
> > +        qobject_decref(data);
> > +    }
> >      object_unparent(OBJECT(dev));
> >  }
> >  
> 
> I'm pretty sure this is the wrong place to fire the notification. We
> should rather do this when the device is actually deleted - which
> qdev_free() does *not* actually guarantee, as criticized in the s390x
> and unref'ing contexts.
> I would suggest to place your code into device_unparent() instead.
> 
> Another thing to consider is what data to pass to the event: Not all
> devices have an ID.

If they don't they were not created by management so management is
probably not interested in them being removed.

We could always add a 'path' key later if this assumption
proves incorrect.

> We should still have a canonical path when we fire
> this event in either qdev_free() or in device_unparent() before the if
> (dev->parent_bus) block though. That would be a question for Anthony,
> not having a use case for the event I am indifferent there.
> 
> Further, thinking of objects such as virtio-rng backends or future
> blockdev/chardev objects, might it make sense to turn this into a
> generic object deletion event rather than a device event?
> 
> Andreas

Backend deletion doesn't normally have guest interaction right?
So why do we need an event?

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