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

Avi Kivity avi at redhat.com
Thu Apr 9 08:19:43 UTC 2009

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.

What's wrong with having 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.

Multiple monitors would still come in useful for debugging in a managed 

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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