Re: [Qemu-devel] monitor: enable OOB by default

[Markus Armbruster <armbru@redhat.com>]

Peter Xu <peterx@redhat.com> writes:

> On Wed, Jun 27, 2018 at 01:23:07PM +0200, Markus Armbruster wrote:
>> Daniel P. Berrangé <berrange@redhat.com> writes:
>> 
>> > On Wed, Jun 27, 2018 at 10:41:38AM +0200, Markus Armbruster wrote:
>> >> Markus Armbruster <armbru@redhat.com> writes:
>> >> 
>> >> > Markus Armbruster <armbru@redhat.com> 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=$HOME/.qmp_history,prompt='QMP> '
>> >> >>     {"QMP": {"version": {"qemu": {"micro": 50, "minor": 12, "major": 2}, "package": "v2.12.0-1703-gb909799463"}, "capabilities": ["oob"]}}
>> >> >>     QMP> { "execute": "qmp_capabilities", "arguments": { "oob": true } }
>> >> >>     {"error": {"class": "GenericError", "desc": "Parameter 'oob' is unexpected"}}
>> >> >>     QMP> { "execute": "qmp_capabilities", "arguments": { "enable": ["oob"] } }
>> >> >>     {"return": {}}
>> >> >>     QMP> { "execute": "query-qmp-schema" }
>> >> >>     {"error": {"class": "GenericError", "desc": "Out-Of-Band capability 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 reason?
>> >> >
>> >> > [...]
>> >> 
>> >> Apropos COMMAND_DROPPED: we send an event rather than an error response
>> >> because we may send it out-of-order.  Makes sense.
>> >> 
>> >> 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 clashing
>> > "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.
>> 
>> Right.  Must fix bug.
>
> Even more true.
>
>> 
>> 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.