On 12/16/2015 04:47 AM, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> Emit an event each time we sync the dirty bitmap on the source;
> this helps libvirt use postcopy by giving it a kick when it
> might be a good idea to start the postcopy.
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  docs/qmp-events.txt | 14 ++++++++++++++
>  migration/ram.c     |  4 ++++
>  qapi/event.json     | 13 +++++++++++++
>  3 files changed, 31 insertions(+)
> 

>  
> +MIGRATION_PASS
> +--------------
> +
> +Emitted from the source side of a migration at the start of each pass
> +(when it syncs the dirty bitmap)
> +
> +Data: None.
> +
> +  - "pass": An incrementing count (starting at 1 on the first pass)
> +
> +Example:
> +{"timestamp": {"seconds": 1449669631, "microseconds": 239225},
> + "event": "MIGRATION_PASS", "data": {"pass": 2}}

How frequently can this event be emitted?  It is not under guest
control, so it's harder to argue that it could be abused as a way for
the guest to DoS the host.  And hopefully it is slow enough (considering
that each pass also requires lots more effort for actually transmitting
state from source to dest), so I don't think we have to worry about
adding rate-limiting to avoid overwhelming the monitor client.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org