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

[Peter Xu <peterx@redhat.com>]

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)

> >
> > 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?

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?

Thanks,

-- 
Peter Xu