Re: [Qemu-devel] [RFC] QEMU Dbus support - a proposal management API

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Dec 11, 2007 at 08:51:32AM -0600, Anthony Liguori wrote:
> Dor Laor wrote:
> >Laurent Vivier wrote:
> >>Le mardi 11 décembre 2007 à 10:10 +0100, Fabrice Bellard a écrit :
> >>  
> >>>Hi,
> >>>    
> >>
> >>Hi,
> >>
> >>  
> >>>At this point I am not interested in integrating it into QEMU as it is 
> >>>one more API level to maintain in addition to the command line monitor. 
> >>>However, I can change my mind if several projects insists to have a 
> >>>similar interface.
> >>>    
> >>
> >>perhaps the DBUS interface can replace the command line monitor ?
> >>We have just to move the command line interface to a client speaking to
> >>qemu through the DBUS interface.
> >>
> >>  
> >This is a valid option but the problem is that local user will have to 
> >use another tool (client) to
> >send commands. Another option is to have a common backend with machine 
> >& user interfaces.
> >For example, if we use dbus as the backend, monitor commands will just 
> >be translated into dbus.
> >The opposite option is also valid.
> >
> >Anyway, the motivation behind a new interface is that the monitor 
> >interface is not good enough for automation:
> >There are not return status for commands, no option for async 
> >notifications, no option for parallel actions in case
> >a command takes long time to complete (like snapshot).
> 
> All of these are valid, and addressable.  Return statuses can just be 
> added to the beginning of the output of each command (similar to how 
> POP3 works).  Async notification can be made to work by add support to 
> the monitor for a "select" command.  Semantically, select would block 
> the monitor and then output events.   For this to work really well, you 
> would have to support multiple simultaneous monitor sessions.  The 
> parallel options for long running commands is already address in KVM 
> with the migration command.  We just have to rework the snapshotting to 
> be properly asynchronous.
> 
> >
> >So we either a new interface is added or the existing one will be 
> >enhanced.
> >Since Qemu/KVM will be used in production its highly important to have 
> >a reliable channel to connects with mgmt daemons.
> >Dbus is a common practice for communication and used in Linux, 
> >libvirt, etc. The question is whether to add a dbus server to Qemu or
> >a client is sufficient.
> 
> The main objection I have to dbus is that it's very heavy weight.  It 
> implies a rather fat infrastructure and it not very suitable for 
> embedding.  QEMU has very few dependencies and that is a strength ATM.  
> People interested in embedding QEMU still want a good management 
> interface so enhancing the monitor seems more preferable to me than 
> adding a dbus dependency.

It is also not so easily portable to other OS like Windows & Mac OS, who
will still be lacking a decent control API. As I mentioned in my other
thread, embedding a DBus in each individual QEMU process is not the right
way to write DBus services either. If you want DBus (which is questionable
in itself), then you want to have a single service which manages all a QEMU
VMs. This mandates that the DBus service be outside the context of the QEMU
process itself.


Dan.
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