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

[Michael S. Tsirkin]

On Thu, Mar 07, 2013 at 08:00:29PM +0100, Andreas Färber wrote:
> Am 07.03.2013 19:12, schrieb Michael S. Tsirkin:
> > On Thu, Mar 07, 2013 at 06:23:46PM +0100, Markus Armbruster wrote:
> >> "Michael S. Tsirkin" <address@hidden> writes:
> >>
> >>> On Thu, Mar 07, 2013 at 03:14:15PM +0100, Markus Armbruster wrote:
> >>>> Andreas Färber <address@hidden> writes:
> >>>>
> >>>>> Am 07.03.2013 11:07, schrieb Michael S. Tsirkin:
> >>>>>> On Thu, Mar 07, 2013 at 10:55:23AM +0100, Markus Armbruster wrote:
> >>>>>>> "Michael S. Tsirkin" <address@hidden> writes:
> >>>>>>>
> >>>>>>>> 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.
> >>>>>>>
> >>>>>>> In old qdev, ID was all we had, because paths were busted.  Thus,
> >>>>>>> management had no choice but use IDs.
> >>>>>>>
> >>>>>>> If I understand modern qdev correctly, we got a canonical path.  Old
> >>>>>>> APIs like device_del still accept only ID.  Should new APIs still be
> >>>>>>> designed that way?  Or should they always accept / provide the 
> >>>>>>> canonical
> >>>>>>> path, plus optional ID for convenience?
> >>>>>>
> >>>>>> What are advantages of exposing the path to users in this way?
> >>>>
> >>>> The path is the device's canonical name.  Canonical means path:device is
> >>>> 1:1.  Path always works.  Qdev ID only works when the user assigned one.
> >>>>
> >>>> Funny case: board creates a hot-pluggable device by default (thus no
> >>>> qdev ID), guest ejects it, what do you put into the event?  Your code
> >>>> simply doesn't emit one.
> >>>>
> >>>> You could blame the user; after all he could've used -nodefaults, and
> >>>> added the device himself, with an ID.
> >>>>
> >>>> I blame your design instead, which needlessly complicates the event's
> >>>> semantics: it gets emitted only for devices with a qdev ID.  Which you
> >>>> neglected to document clearly, by the way.
> >>>
> >>> Good point, I'll document this.
> >>>
> >>>> If you put the path into the event, you can emit it always, which is
> >>>> simpler.  Feel free to throw in the qdev ID.
> >>>
> >>> I don't blame anyone.  User not assigning an id is a clear indication
> >>> that user does not care about the lifetime of this device.
> >>>
> >>>>>> Looks like maintainance hassle without real benefits?
> >>>>
> >>>> I can't see path being a greater maintenance hassle than ID.
> >>>
> >>> Sure, the less events we emit the less we need to support.
> >>> You want to expose all kind of internal events,
> >>> then management will come to depend on it and
> >>> we'll have to maintain them forever.
> >>
> >> Misunderstanding.  I'm *not* asking for more events.  I'm asking for the
> >> DEVICE_DELETED event to carry the device's canonical name: its QOM path.
> >>
> >>>>> Anthony had rejected earlier QOM patches by Paolo related to qdev id,
> >>>>> saying it was deprecated in favor of those QOM paths.
> >>>>
> >>>> More reason to put the path into the event, not just the qdev ID.
> >>>
> >>> libvirt does not seems to want it there. We'll always be able to
> >>> add info but will never be able to remove info, keep it minimal.
> >>
> >> Yes, adding members to an event is easy.  Doesn't mean we should do it
> >> just for the heck of it.  If we don't need a member now, and we think
> >> there's a chance we won't need in the future, then we probably shouldn't
> >> add it now.
> >>
> >> I believe the chance of not needing the QOM path is effectively zero.
> >>
> >> Moreover, we'd add not just a member in this case, we'd add a *trigger*.
> >>
> >> Before: the event gets emitted only for devices with a qdev ID.
> >>
> >> After: the event gets emitted for all devices.
> >>
> >> I very much prefer the latter, because it's simpler.
> >>
> >> [...]
> > 
> > I still don't see why it's useful for anyone.  For now I hear from the
> > libvirt guys that this patch does exactly what they need so I'll keep it
> > simple.  You are welcome to send a follow-up patch adding a path
> > and more triggers, I won't object.
> 
> Well, the libvirt guys have been told to poll using qom-list, which
> needs the path, not an ID. Using it in both places would make it
> symmetrical - that may qualify as useful.
> (I'm not aware of any id -> path lookup QMP command.)
> 
> Nontheless, you can retain my Reviewed-by on v4+ as long as the code in
> hw/qdev.c doesn't change.
> 
> Andreas

I suggested retrying device_del, this has an advantage of working
on more qemu version.

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