[Qemu-devel] Re: [PATCH 2/3] Introduce monitor 'wait' command

[Anthony Liguori <aliguori@us.ibm.com>]

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