[libvirt] Re: [Qemu-devel] [PATCH 1/6] Allow multiple monitor devices (v2)

Anthony Liguori aliguori at us.ibm.com
Thu Apr 9 13:28:39 UTC 2009

Avi Kivity wrote:
> Anthony Liguori wrote:
>> Right now only one monitor device can be enabled at a time.  In order 
>> to support
>> asynchronous notification of events, I would like to introduce a 
>> 'wait' command
>> that waits for an event to occur.  This implies that we need an 
>> additional
>> monitor session to allow commands to still be executed while waiting 
>> for an
>> asynchronous notification.
> I think this is race prone.  For example:
>  monitor 1: wait
>  monitor 2: hotunplug dev1
>  monitor 2: hotplug dev1
>  monitor 1: event
> there is no way to tell whether event (which relates to dev1) happened 
> the hotunplug or after the hotunplug.  In general there is no way to 
> correlate events to commands.

events don't correlate to commands.  Can you give an example of why 
you'd want to correlate and event to a command.

> What's wrong with having async notifications?

Isn't this async notifications?

>   Sure, we'll need to make sure notifications don't mix with command 
> responses, and that all commands are acked (the (qemu) prompt serves 
> that purpose now), but it seems to me do be a lot easier for the 
> management end.

FWIW, you can have asynchronous completion of monitor commands.  See 
migration as an example.  The (qemu) prompt is the ack that the command 
is finished.  You can only have one async command per monitor session 
which is why you need multiple monitors.

If we had a non-human monitor mode, I think it would be fine to have 
many monitors multiplexed over a single channel.  The internal monitor 
abstraction doesn't change.

I think multiplexing multiple monitor sessions requires a non-human mode 
because you need to tag all output, etc. which is what we need for 
non-human mode anyway.


Anthony Liguori

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