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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [RFC] QEMU Dbus support - a proposal management API
Date: Tue, 11 Dec 2007 09:36:47 -0600
User-agent: Thunderbird (X11/20071022)

Yuval Kashtan wrote:
As I can see,
There is HUGH interest in management API for QEMU.
seemly, DBus is NOT the right solution for direct integration into QEMU as it is not cross platform enough, pose extra dependency and (probably) not suitable for embedded systems.

Keeping only the "old" monitor interface with no formal interface will make QEMU harder to integrate with as it requires extra work

Why can't we make the monitor interface a "formal" interface?


Anthony Liguori

C API is not a complete solution as it does not define well enough how will you integrate with QEMU without changing or adding to the source.. But combined with AVI's idea of some plug-in mechanism, which will use this C API, it sounds like a complete and valid solution.

The monitor should be kept for backward compatibility, human interface and be implemented on top of that API as well, so there will also be a reference implementation of how to use the API and create a plug-in.

As of Daniel remarks regarding DBus:
- I actually have a newer version where commands are divided into different DBus interfaces. So (hopefully) that solves the methods naming problem. - I had to pass the server name at the CLI, since I was executing multiple instances of QEMU.. Having an external server, raises again the question of how will that server communicate with each instance of QEMU? - It was easier for my project to configure QEMU after it started and not through a file, that why there is a configuration method for each option. of course there could also be a bulk method which receive them all or even read them from a file (and actually with some extra work, many of them could be made dynamics)

On Dec 11, 2007 5:00 PM, Daniel P. Berrange <address@hidden <mailto:address@hidden>> wrote:

    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
    > >& 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
    > 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
    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.

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Yuval Kashtan

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