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

[Eric Blake <eblake@redhat.com>]

On 06/28/2018 01:55 AM, Markus Armbruster wrote:

> 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.

And no one could have been relying on the broadcast COMMAND_DROPPED 
event semantics, since OOB was previously experimental and since we just 
proved that they are wrong if not limited to one monitor.

> 
> 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.

Also, fewer NULL arguments to add into existing call sites (although 
existing call sites would change to use the _bcast_ form, so you're 
already touching all call sites, which brings back the topic from the 
other mail on dropping useless errp while at it).

> 
> 
> [*] 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.

Indeed, and I thought the conclusion there was that we DO promise that 
COMMAND_DROPPED (and dropped commands in general) won't happen unless 
you negotiate oob at initial connection.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org