Re: [Qemu-devel] [PATCH 0/8]: QMP feature negotiation support

[Markus Armbruster]

Luiz Capitulino <address@hidden> writes:

>  Feature negotiation allows clients to enable new QMP capabilities they
> support and thus allows QMP to envolve in a compatible way.
>
>  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.

Well, it becomes part of the protocol then.  But I understand what you
mean.

>  Feature negotiation is implemented by, among other changes, adding
> mode-oriented support to QMP, which comprehends the following:
>
> o Two modes: handshake and operational
> o All QMP Monitors start in handshake mode
> o In handshake mode only commands to query/enable/disable QMP capabilities are
>   allowed (there are few exceptions)
> o Clients can switch to the operational mode at any time
> o In Operational mode most commands are allowed and QMP capabilities changes
>   made in handshake mode take effect
>
>  Please, note that we don't have any capability yet. So, the most visable
> change in this series is that now Clients must switch to operational mode to
> be able to issue 'regular' commands.
>
>  Session example:
>
> """
> {"QMP": {"capabilities": []}}
>
> { "execute": "query-qmp-mode" }
> {"return": {"mode": "handshake"}}
>
> { "execute": "stop" }
> {"error": {"class": "CommandNotFound", "desc": "The command stop has not been 
> found", "data": {"name": "stop"}}}
>
> { "execute": "qmp_capability_enable", "arguments": { "name": "foobar" } }
> {"error": {"class": "InvalidParameter", "desc": "Invalid parameter name", 
> "data": {"name": "name"}}}
>
> { "execute": "qmp_switch_mode", "arguments": { "mode": "operational" } }
> {"return": {}}
>
> { "execute": "query-qmp-mode" }
> {"return": {"mode": "operational"}}
>
> { "execute": "stop" }
> {"return": {}}
>
> """

I don't doubt your design does the job.  I just think it's overly
general.  I had something far more stupid in mind:

    client connects
    server -> client: version & capability offer (one message)
  again:
    client -> server: capability selection (one message)
    server -> client: either okay or error (one message)
    if error goto again
    connection is now ready for commands

No modes.  The distinct lack of generality is a design feature.