From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:38485) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UDfIm-0003tK-5S for qemu-devel@nongnu.org; Thu, 07 Mar 2013 13:12:28 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UDfIc-0003Ip-St for qemu-devel@nongnu.org; Thu, 07 Mar 2013 13:12:24 -0500 Received: from mx1.redhat.com ([209.132.183.28]:8289) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UDfIc-0003Ig-LH for qemu-devel@nongnu.org; Thu, 07 Mar 2013 13:12:14 -0500 Date: Thu, 7 Mar 2013 20:12:29 +0200 From: "Michael S. Tsirkin" Message-ID: <20130307181229.GB30633@redhat.com> References: <20130306130055.GA29446@redhat.com> <51374B42.2090405@suse.de> <20130306141324.GA30001@redhat.com> <87y5dzlek4.fsf@blackfin.pond.sub.org> <20130307100740.GB5302@redhat.com> <5138921D.5050604@suse.de> <87ehfrcn60.fsf@blackfin.pond.sub.org> <20130307163540.GB29071@redhat.com> <87k3pj3yzh.fsf@blackfin.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <87k3pj3yzh.fsf@blackfin.pond.sub.org> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH] qdev: DEVICE_DELETED event List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: Kevin Wolf , Anthony Liguori , Eduardo Habkost , Stefan Hajnoczi , qemu-devel@nongnu.org, Luiz Capitulino , Gerd Hoffmann , laine@redhat.com, Paolo Bonzini , Andreas =?iso-8859-1?Q?F=E4rber?= On Thu, Mar 07, 2013 at 06:23:46PM +0100, Markus Armbruster wrote: > "Michael S. Tsirkin" writes: >=20 > > On Thu, Mar 07, 2013 at 03:14:15PM +0100, Markus Armbruster wrote: > >> Andreas F=E4rber writes: > >>=20 > >> > 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" writes: > >> >>> > >> >>>> On Wed, Mar 06, 2013 at 02:57:22PM +0100, Andreas F=E4rber wrot= e: > >> >>>>> 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 > >> >>>>> > >> >>>>> 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" > >> >>>>>> =20 > >> >>>>>> int qdev_hotplug =3D 0; > >> >>>>>> static bool qdev_hot_added =3D 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 =3D qobject_from_jsonf("{ 'device': %s= }", dev->id); > >> >>>>>> + monitor_protocol_event(QEVENT_DEVICE_DELETED, data); > >> >>>>>> + qobject_decref(data); > >> >>>>>> + } > >> >>>>>> object_unparent(OBJECT(dev)); > >> >>>>>> } > >> >>>>>> =20 > >> >>>>> > >> >>>>> I'm pretty sure this is the wrong place to fire the notificati= on. We > >> >>>>> should rather do this when the device is actually deleted - wh= ich > >> >>>>> qdev_free() does *not* actually guarantee, as criticized in th= e s390x > >> >>>>> and unref'ing contexts. > >> >>>>> I would suggest to place your code into device_unparent() inst= ead. > >> >>>>> > >> >>>>> Another thing to consider is what data to pass to the event: N= ot 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 stil= l be > >> >>> designed that way? Or should they always accept / provide the c= anonical > >> >>> path, plus optional ID for convenience? > >> >>=20 > >> >> What are advantages of exposing the path to users in this way? > >>=20 > >> The path is the device's canonical name. Canonical means path:devic= e is > >> 1:1. Path always works. Qdev ID only works when the user assigned = one. > >>=20 > >> 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 cod= e > >> simply doesn't emit one. > >>=20 > >> You could blame the user; after all he could've used -nodefaults, an= d > >> added the device himself, with an ID. > >>=20 > >> I blame your design instead, which needlessly complicates the event'= s > >> semantics: it gets emitted only for devices with a qdev ID. Which y= ou > >> 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? > >>=20 > >> 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. >=20 > Misunderstanding. I'm *not* asking for more events. I'm asking for th= e > DEVICE_DELETED event to carry the device's canonical name: its QOM path. >=20 > >> > Anthony had rejected earlier QOM patches by Paolo related to qdev = id, > >> > saying it was deprecated in favor of those QOM paths. > >>=20 > >> 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. >=20 > 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. >=20 > I believe the chance of not needing the QOM path is effectively zero. >=20 > Moreover, we'd add not just a member in this case, we'd add a *trigger*. >=20 > Before: the event gets emitted only for devices with a qdev ID. >=20 > After: the event gets emitted for all devices. >=20 > I very much prefer the latter, because it's simpler. >=20 > [...] 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. --=20 MST