[Qemu-devel] QMP forward compatibility support

[Luiz Capitulino <lcapitulino@redhat.com>]

 Hi.

 We (Markus and I) are working on getting QMP forward compatibility support,
supported. :)

 We have a plan for it and I'd like to ask the CC'ed people to review it.

 Needless to say, but the objective here is to add new commands, arguments,
async messages and protocol features w/o breaking existing clients.

General Plan
------------

1. QMP should describe itself, ie. it should dump all accepted commands,
their replies and arguments, async messages and protocol features. All in
JSON format

2. Protocol features are advertised by the (already existent) capabilities
array, but are _disabled_ by default. The exception is async messages,
which are _enabled_ by default

3. We should add command(s) to enable/disable protocol features

4. Proper feature negotiation is done in pause mode. That's, clients
interested in enabling new protocol features should start QEMU in
pause mode and enable the features they are interested in using

Work to do
----------

1. Markus is already working on getting QMP to describe itself, we
should get a status/proposal soon. This is the most important part,
as we may want to change query-commands output, query-events is
missing and hopefully we can generate user docs from this

2. We need to add commands to enable/disable features. Not high priority,
as the only existing feature is async messages which are enabled by
default