From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45683) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fYQqR-0008H6-JL for qemu-devel@nongnu.org; Thu, 28 Jun 2018 02:55:56 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fYQqO-0006sf-FA for qemu-devel@nongnu.org; Thu, 28 Jun 2018 02:55:55 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:38688 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fYQqO-0006qZ-8w for qemu-devel@nongnu.org; Thu, 28 Jun 2018 02:55:52 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6A1D640255D7 for ; Thu, 28 Jun 2018 06:55:51 +0000 (UTC) From: Markus Armbruster References: <20180620073223.31964-1-peterx@redhat.com> <871sctea4y.fsf@dusky.pond.sub.org> <87tvpoadcc.fsf_-_@dusky.pond.sub.org> <87wouk8vul.fsf@dusky.pond.sub.org> <20180627102043.GD30628@redhat.com> <8760248odg.fsf@dusky.pond.sub.org> <20180627120733.GD2516@xz-mi> Date: Thu, 28 Jun 2018 08:55:48 +0200 In-Reply-To: <20180627120733.GD2516@xz-mi> (Peter Xu's message of "Wed, 27 Jun 2018 20:07:33 +0800") Message-ID: <877emj5rij.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] monitor: enable OOB by default List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Xu Cc: Markus Armbruster , qemu-devel@nongnu.org Peter Xu writes: > On Wed, Jun 27, 2018 at 01:23:07PM +0200, Markus Armbruster wrote: >> Daniel P. Berrang=C3=A9 writes: >>=20 >> > On Wed, Jun 27, 2018 at 10:41:38AM +0200, Markus Armbruster wrote: >> >> Markus Armbruster writes: >> >>=20 >> >> > Markus Armbruster writes: >> >> > >> >> >> I fooled around a bit, and I think there are a few lose ends. >> >> > [...] >> >> >> Talking to a QMP monitor that supports OOB: >> >> >> >> >> >> $ socat UNIX:test-qmp READLINE,history=3D$HOME/.qmp_history,pr= ompt=3D'QMP> ' >> >> >> {"QMP": {"version": {"qemu": {"micro": 50, "minor": 12, "major= ": 2}, "package": "v2.12.0-1703-gb909799463"}, "capabilities": ["oob"]}} >> >> >> QMP> { "execute": "qmp_capabilities", "arguments": { "oob": tr= ue } } >> >> >> {"error": {"class": "GenericError", "desc": "Parameter 'oob' i= s unexpected"}} >> >> >> QMP> { "execute": "qmp_capabilities", "arguments": { "enable":= ["oob"] } } >> >> >> {"return": {}} >> >> >> QMP> { "execute": "query-qmp-schema" } >> >> >> {"error": {"class": "GenericError", "desc": "Out-Of-Band capab= ility requires that every command contains an 'id' field"}} >> >> >> >> >> >> Why does every command require 'id'? >> >> > >> >> > I found one reason: event COMMAND_DROPPED wants it. Any other reas= on? >> >> > >> >> > [...] >> >>=20 >> >> Apropos COMMAND_DROPPED: we send an event rather than an error respon= se >> >> because we may send it out-of-order. Makes sense. >> >>=20 >> >> However, broadcasting it to all monitors doesn't make sense. We could >> >> use a way to send an event to just one monitor. > > True. > > (Sorry for the late responses; I was on Linuxcon China in the past few > days) No need to be sorry :) >> > >> > Worse than that - broadcasting to all monitors is categorically broken. >> > Different monitors make use the same "id" formatting scheme, so if you >> > broadcast COMMAND_DROPPED to a different monitor you might have clashi= ng >> > "id" and thus incorrectly tell a client its command was dropped when in >> > fact it was processed. You'd have to be fairly unlucky in timing, but >> > it could happen. >>=20 >> Right. Must fix bug. > > Even more true. > >>=20 >> I'm glad I went over this one more time, and in public! > > I had a glance at current qmp-spec, it seems that we don't have any > restriction currently on "we must send events to all the monitors". > Does it mean that we should be free to have per-monitor events > starting from this event? Changing an existing event from broadcast to unicast is an observable change in existing behavior. Compatibility break unless we can show nobody can use / uses the observation. Creating a new event is not a compatibility break by itself[*], regardless of broadcast vs. unicast. > My current plan is that I can touch up scripts/qapi/events.py and > related stuff to allow QMPEventFuncEmit to take a monitor parameter, > then we pass in NULL when we want to send the event to all monitors. > > Would that work? Think so. Alternatively, a pair of functions: void qapi_event_bcast_EVENT(... event params ..., Error **errp); void qapi_event_send_EVENT(Monitor *mon, ... event params ..., Error **= errp); Slightly more compact in the broadcast case, might be a bit easier to read. [*] In the case of COMMAND_DROPPED, the compatibility break is dropping commands (the event's trigger), caused by the command queuing feature. That's why command queuing has to be opt-in. Details discussed elsewhere in this thread.