On Wed, 13 Jan 2010 17:53:38 +0100 Markus Armbruster <arm...@redhat.com> wrote:
> Luiz Capitulino <lcapitul...@redhat.com> writes: > > > On Mon, 11 Jan 2010 18:24:24 -0600 > > Anthony Liguori <anth...@codemonkey.ws> wrote: > > > >> On 01/11/2010 06:04 PM, Luiz Capitulino wrote: > >> > > >> > As async messages were one of the reasons for having QMP, I thought > >> > that there was a consensus that making it part of the "original" > >> > protocol was ok, meaning that they would be always available. > >> > > >> > That's the only reason. > >> > > >> > >> Right, but then it's not a capability, it's a core feature. I just > >> think it would be odd to advertise something as a capability and have it > >> not behave like other ones. > > > > Ok, so options are: call it a core feature and don't change anything > > or call it a capability and make it behave like any other capability. > > > > What's the better? Should we vote? :) Daniel seems to prefer the > > later. > > If it's optional, leave it off by default because the consensus seems to > be to leave all optional features off by default. > > It should be optional if we want to support clients that don't want it. > I don't think coping with it would be a terrible burden on clients, but > neither is having to ask for it. Personally, I'd make it optional. Ok, will do. > >> >>> 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 > >> >>> > >> >>> > >> >> Why does this matter? > >> >> > >> >> We should be careful to support connecting to a VM long after it's been > >> >> started so any requirement like this is likely to cause trouble. > >> >> > >> > If I understood Markus's concerns correctly, he thinks that feature > >> > negotiation should happen before the protocol is "running", ie. make > >> > it part of the initial handshake. > >> > > >> > >> I think forcing the negotiation before executing any commands is a good > >> idea. But I don't think requiring the guest not to be running is > >> necessary or even useful. > >> > >> You don't want to have to support disabling and enabling features in the > >> middle of a protocol session because then you have to deal with weird > >> semantics. > > > > That's true, but I thought that doing that with pause mode was > > going to be better because it didn't require any change on QMP side. > > > > If this is a bad approach, then I was wrong. > > > > Now, making this part of the initial handshake brings some more > > design decisions and by making async messages a capability helps > > to test them. > > > > I'm thinking in something like this: > > > > 1. Connection is made, the greeting message is sent and QMP is > > in 'handshake mode' > > > > 2. In this mode only commands to enable/disable protocol > > capabilities are allowed > > > > 3. When the client is done with the setup, it issues the > > command 'enable-qmp', which puts the protocol into 'running mode', > > where any command is accepted > > Really "any command"? What about commands to enable/disable protocol > capabilities? I think that playing with some protocol bits might be safe, like enabling async messages. I'm not saying this is a good practice, but forbidding it seems a bit extreme at first.
