[libvirt] qemu: domain notification for serial channel changes

[Matt Broadstone]

On Mon, Nov 7, 2016 at 8:52 AM, Matt Broadstone <mbroadst at gmail.com> wrote:
> On Mon, Nov 7, 2016 at 8:35 AM, Peter Krempa <pkrempa at redhat.com> wrote:
>> 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
>
> Daniel, Peter,
>
> Thanks for your quick responses. I have a few questions before implementation:
>
>   - I assume that you would prefer to keep the existing guest
> lifecycle events, for backwards compatibility?
>
>   - Do you have a preference wrt naming this event? I see some
> inconsistencies the events ids (design-wise) (e.g.
> VIR_DOMAIN_EVENT_ID_DEVICE_ADDED  / VIR_DOMAIN_EVENT_ID_DEVICE_REMOVED
> / VIR_DOMAIN_EVENT_ID_DEVICE_DEVICE_REMOVAL_FAILED, instead of a
> single event for all potential changes like
> VIR_DOMAIN_EVENT_ID_AGENT_LIFECYCLE).
>
> Matt

Ah, sorry one followup question. Should the event generalize to serial
changes, or is the preference to keep it specific to channels?

Matt