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[libvirt] Re: [PATCH 2/3] Introduce monitor 'wait' command



Daniel P. Berrange wrote:
On Wed, Apr 08, 2009 at 09:16:43AM -0500, Anthony Liguori wrote:
The wait command will pause the monitor the command was issued in until a new
event becomes available.  Events are queued if there isn't a waiter present.
The wait command completes after a single event is available.

Today, we queue events indefinitely but in the future, I suspect we'll drop
events that are older than a certain amount of time to avoid infinitely
allocating memory for long running VMs.

To make use of the new notification mechanism, this patch introduces a
qemu_notify_event() API.  This API takes three parameters: a class which is
meant to classify the type of event being generated, a name which is meant to
distinguish which event in the class that has been generated, and a details
parameters which is meant to allow events to send arbitrary data with a given
event.

Perhaps we should have the ability to turn on/off events, via a 'notify EVENT'
command, and a way turn off the prompt on the channel used for receiving
events.

So if I was interested in RTC change, and VNC client connection events, on
the main monitor command channel we'd do:

  (qemu)  notify rtc-change
  rtc-change notification enabled
  (qemu)  notify vnc-client
  vnc-client notification enabled
  (qemu)

So you want to mask out event types? I think you could do this with the actual wait command either inclusively:

(qemu) wait "rtc-change vnc-client"
...

Or exclusively:

(qemu) wait -x "rtc-change vnc-client"
...

And then in the 2nd monitor channel, a single 'wait' command would turn
off the monitor prompt and make the channel dedicated for just events,
one per line

  (qemu) wait
  rtc-change UTC+0100
  vnc-client connect 192.46.12.4:9353
  vnc-client disconnect 192.46.12.4:9353
  vnc-client connect 192.46.12.2:9353
  vnc-client disconnect 192.46.12.2:9353

N.B. Right now, wait returns only a single event. This is because the output format is:

(qemu) wait
1239200822.748241: vm-state: stop
(qemu)

But vm-state doesn't have any details, if it had details it would be:

(qemu) wait
1239200822.748241: vm-state: stop
The virtual machine has stopped.
(qemu)

Since everyone already parses commands like this, I think the format makes sense. It implies that the event dispatch code has to sit constantly issuing wait commands.

In my next version of the patch, I expire old events (older than 10 minutes), and also add a -d flag to poll for events vs. wait.

Regards,

Anthony Liguori

Daniel


--
Regards,

Anthony Liguori


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