Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 22 Jan 2010 11:21:22 +0100
Markus Armbruster <armbru@redhat.com> wrote:

> A few quick questions before I dive into the patches...
> 
> Luiz Capitulino <lcapitulino@redhat.com> writes:
> 
> >  Feature negotiation allows clients to enable QMP capabilities they are
> > interested in using. This allows QMP to envolve without breaking old clients.
> >
> >  A capability is a new QMP feature and/or protocol change which is not part of
> > the core protocol as defined in the QMP spec.
> >
> >  Feature negotiation is implemented by defining a set of rules and adding
> > mode-oriented support.
> >
> >  The set of rules are:
> >
> > o All QMP capabilities are disabled by default
> > o All QMP capabilities must be advertised in the capabilities array
> > o Commands to enable/disable capabilities must be provided
> >
> > NOTE: Asynchronous messages are now considered a capability.
> >
> >  Mode-oriented support adds the following to QMP:
> >
> > o Two modes: handshake and operational
> > o By default all QMP Monitors start in handshake mode
> 
> "By default"?  Is there a way to start a QMP monitor in another mode?

 No, you think it's worth or is it just about the English?

> > o In handshake mode only commands to query/enable/disable QMP capabilities are
> >   allowed (there are few exceptions)
> 
> Note to self: check out the exception, and why we might want them.

 The following handlers are handshake-only:

- qmp_switch_mode
- async_msg_enable
- async_msg_disable

 The following handlers are allowed to run on both modes:

- query-version
- query-qmp-mode
- query-commands

 Also, all the self-description commands (query-async-msg,
query-errors etc) would be allowed on both modes.

 So, the only handler which is not completely related to feature
negotiation is query-version. This is only a guess, but I think
it might be worth to let clients learn the QEMU version they are
talking to before setting protocol features.

> > o Clients can switch to the operational mode at any time
> 
> Can they switch back?  I hope not.

 No, they can't. The only transition allowed is handshake -> operational.

> > o In Operational mode most commands are allowed and QMP capabilities changes
> >   made in handshake mode take effect
> >
> >  Also note that each QMP Monitor has its own mode state and set of capabilities,
> > this means that if QEMU is started with N QMP Monitors protocol setup done in
> > one of them doesn't affect the others.
> >
> >  Session example:
> >
> > """
> > {"QMP": {"capabilities": ["async messages"]}}
> >
> > { "execute": "query-qmp-mode" }
> > {"return": {"mode": "handshake"}}
> 
> Why would clients want to query the mode?

 It's more useful for testing purposes.

> > { "execute": "change", "arguments": { "device": "vnc", "target": "password", "arg": "1234" } }
> > {"error": {"class": "QMPInvalidModeCommad", "desc": "The issued command is invalid in this mode", "data": {}}}
> 
> I'd treat this like a completely unknown command.

 Really? This would simplify things a bit.

> > { "execute": "async_msg_enable", "arguments": { "name": "STOP" } }
> > {"return": {}}
> >
> > { "execute": "qmp_switch_mode", "arguments": { "mode": "operational" } }
> > {"return": {}}
> 
> Do we envisage mode transitions other than handshake -> operational?

 Today we don't, but this is about forward compatibility support right? :)

 So, IMO it makes sense to have a more general command instead of
qmp_switch_operational.