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Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support
From: |
Luiz Capitulino |
Subject: |
Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support |
Date: |
Fri, 22 Jan 2010 10:09:20 -0200 |
On Fri, 22 Jan 2010 11:21:22 +0100
Markus Armbruster <address@hidden> wrote:
> A few quick questions before I dive into the patches...
>
> Luiz Capitulino <address@hidden> 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.
- Re: [Qemu-devel] [PATCH 08/11] QMP: Asynchronous messages enable/disable support, (continued)
[Qemu-devel] Re: [PATCH 08/11] QMP: Asynchronous messages enable/disable support, Avi Kivity, 2010/01/24
[Qemu-devel] [PATCH 09/11] Monitor: Introduce find_info_cmd(), Luiz Capitulino, 2010/01/21
[Qemu-devel] [PATCH 10/11] QError: New QERR_QMP_INVALID_MODE_COMMAND, Luiz Capitulino, 2010/01/21
[Qemu-devel] [PATCH 11/11] QMP: Enable feature negotiation support, Luiz Capitulino, 2010/01/21
Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support, Markus Armbruster, 2010/01/22
- Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support,
Luiz Capitulino <=
Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support, Anthony Liguori, 2010/01/22
- Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support, Markus Armbruster, 2010/01/25
- Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support, Luiz Capitulino, 2010/01/26
- Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support, Jamie Lokier, 2010/01/26
- Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support, Luiz Capitulino, 2010/01/26
- Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support, Daniel P. Berrange, 2010/01/26
- Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support, Jamie Lokier, 2010/01/26
- Re: [Qemu-devel] [RFC 00/11]: QMP feature negotiation support, Luiz Capitulino, 2010/01/26