[Libvir] Re: Asynchronous notifications of domain start/stop

[Richard W.M. Jones]

Gerd Hoffmann wrote:
> xenstore doesn't calls you back.  xenstore gives you a file handle you
> can select() on and a function you can call when data is available to
> figure which watch did fire.

Yes, my mistake.  I was -- cough -- reading the kernel interface to 
xenbus, not the xenstore userspace interface.  As you rightly say the 
userspace interface gives you a fd which you have to select() on, see 
the C example here: http://wiki.xensource.com/xenwiki/XenStoreReference

>> We would also need to avoid mixing ordinary request/response with the
>>  notifications
> 
> Why?  Just disallowing notifications while a request is in flight should
> be enougth I think.
> 
> I'd like to have something which integrates nicely with the usual
> select() based event loops, like the xenstore model.  Sort of "here is a
> file handle, and if data comes in call this function."  The function in
> turn could either call callbacks or return some event struct.

I can see this working.

In the Xen (local) case we can hand back the Xenstore fd itself for 
applications to monitor.

In the Xen remote case the server side has already got an event loop 
which can look for fd events.  They'll be sent synchronously to the 
client.  The client will have to hand out the remote connection fd for 
applications to select() on.

In the qemu case I'm not sure how to modify qemu_driver.c to handle 
this.  One for Dan Berrange to comment on.

Rich.

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