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Re: [libvirt] [PATCH] add a default event handle, to passthough the new events come from qemu

On Thu, Oct 13, 2011 at 10:54:13AM +0100, Daniel P. Berrange wrote:
> On Tue, Oct 11, 2011 at 08:46:06AM -0500, Adam Litke wrote:
> > On Mon, Oct 10, 2011 at 11:54:08PM +0800, shaohef linux vnet ibm com wrote:
> > > From: Shaohe Feng <shaohef linux vnet ibm com>
> > > 
> > > Basically, this feature can go along with qemu monitor passthrough.
> > > That way, if we use new commands in the monitor that generate new events, we want
> > >  some way to receive those new events too.
> > 
> > I agree that this patch is very complimentary to the existing tools for qemu
> > interaction (qemu_monitor_command, qemu_attach, etc).  It allows API users to
> > subscribe to events that are not yet handled by libvirt.  I have a couple of
> > design questions about this feature.
> > 
> > 1) This feature seems to be a bit qemu specific.  Other qemu-specific APIs
> > (mentioned above) are build into libvirt-qemu.so so that they are kept apart
> > from the hypervisor-neutral parts of the API.  Is it possible to do that for
> > an event handler like this patch implements?  Do we want to enforce such a
> > limit?
> Yep, I agree it ought to be in the QEMU specific library.
> > 2) This patch causes a string representing a raw JSON event object to be
> > passed to the callbacks that are registered for the default event.  This seems
> > fine to me.  I do wonder if relying on a 'default' event is a bad thing for an
> > application to do.  Let's say an app decides to handle NEW_EVENT using this
> > default handler.  Then, libvirt gains official support for NEW_EVENT.  When the
> > libvirt package is updated in the application's environment, NEW_EVENT will no
> > longer trigger via the default handler.  Thus, the application is broken by a
> > libvirt upgrade.  Would it be better to have a 'raw event' sink where all events
> > (whether supported or not) would be sent?
> I expect applications will want the event to be dispatched to them,
> regardless of whether libvirt has handled it itself, so that they
> don't have to update themselves to the latest libvirt event API
> immediately.
> Also I think it would be better if we were able to connect to explicit JSON
> events, rather than a catch all. THe number of events from QEMU is only
> going to increase over time & as it does, a 'catch all' becomes increasingly
> bandwidth wasteful.

Agreed on both of these.

> I'd reckon we want something like this:
>       typedef void (*virConnectDomainQemuEventCallback)(virConnectPtr conn,
>                                                         virDomainPtr dom,
>                                                         const char *eventName, /* The JSON event name */
>                                                         const char *eventArgs, /* The JSON string of args */
>                                                         void *opaque);
>       virConnectDomainQemuEventRegister(virConnectPtr conn,
>                                         virDomainPtr dom, /* option to filter */
>                                         const char *eventName,  /* JSON event name */
>                                         virConnectDomainQemuEventCallback cb,
>                                         void *opaque,
>                                         virFreeCallback freecb);
>       virConnectDomainQemuEventDeregister(virConnectPtr conn,
>                                           int callbackID);
> in libvirt-qemu.h, and libvirt-qemu.so

Looks good to me.

Adam Litke <agl us ibm com>
IBM Linux Technology Center

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