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

[Markus Armbruster]

"Michael S. Tsirkin" <address@hidden> writes:

> 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're keeping it simple only in the sense of keeping it as it is.  I
think it's not as simple as it easily could be.  Specifically, I want
you to simplify the event's trigger, and add a path member.  I'm
convinced we'll have to do this anyway, so why not do it right right
away, and simplify the future.

This is based on my limited understanding of qdev and QOM.  If the QOM
folks tell me that we shouldn't use QOM paths to name the device, I'll
gladly admit to be wrong, and retract my request.

>          You are welcome to send a follow-up patch adding a path
> and more triggers, I won't object.

The usual protocol for patch submission is maintainer (that would be me,
until Luiz is back) reviews, submitter (that would be you) addresses
review comments, within reason.

I believe my request is well within reason.  As a patch submitter, I've
jumped through hoops I found less reasonable (to put it charitably) many
times.

You, as a maintainer, should know that asking the maintainer to address
his review comments himself doesn't scale.

If you think I'm an unreasonable maintainer, feel free to ignore me and
try again when Luiz is back.