[libvirt] qemu: domain notification for serial channel changes

[Peter Krempa]

On Mon, Nov 07, 2016 at 13:31:30 +0000, Daniel Berrange wrote:
> On Mon, Nov 07, 2016 at 08:22:59AM -0500, Matt Broadstone wrote:
> > Hi,
> > 
> > I was in the process of implementing a custom qemu agent which
> > communicates over virtio-serial channels, when I noticed that the only
> > way to receive channel state change notifications was to poll the
> > domstatus XML file. There does seem to be code in libvirt to monitor
> > for serial change events
> > (https://libvirt.org/git/?p=libvirt.git;a=blob;f=src/qemu/qemu_driver.c;h=38c841420e325b6227b3bcd67a26a9b9560b487f;hb=HEAD#l4402),
> > however it's hardcoded/specific to the qemu-ga (explicitly checking
> > for ports named "org.qemu.guest_agent.0").
> > 
> > It occurs to me that while there was a class of events added for guest
> > lifecycle changes, it might be a more generic solution to provide
> > events for character device changes. Am I missing some existing
> > functionality here?  I'm a first-time poster here so I was hoping to
> > both signal my intent to implement something more generic, as well as
> > ping the community to see if there would be interest in this work or
> > reasons why I shouldn't be doing this.
> 
> Yes, having that previous event tied to just the QEMU guest agent looks
> like a mistake to me. We should have a general lifecycle evnt for
> channel devices. It would basically be the same as the guest agent
> lifecycle event, but would include the port name, so you can distinguish
> events from different channels.

That choice was deliberate. I planned to add a separate event for
channels but never got around to do it actually. The guest agent event
is separate since libvirt can have different handling regarding the
guest agent state and I did not want to mix them.

Peter
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