On Thu, Mar 07, 2013 at 09:18:48PM +0100, Markus Armbruster wrote: > "Michael S. Tsirkin" <m...@redhat.com> writes: > > > On Thu, Mar 07, 2013 at 06:23:46PM +0100, Markus Armbruster wrote: > >> "Michael S. Tsirkin" <m...@redhat.com> writes: > >> > >> > On Thu, Mar 07, 2013 at 03:14:15PM +0100, Markus Armbruster wrote: > >> >> Andreas Färber <afaer...@suse.de> 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" <m...@redhat.com> 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 <m...@redhat.com> > >> >> >>>>> > >> >> >>>>> 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.
I think it's a reasonable request, just not sure it's the right thing to do and don't see why it should block this patch when we can add functionality in a follow-up patch. -- MST