Re: [PATCH]: Add a "migrate_incoming" monitor option

[Anthony Liguori <anthony@codemonkey.ws>]

Chris Lalancette wrote:
> We've been trying to plumb libvirt to do KVM migration.  One of the stumbling
> blocks we are running into, however, is that libvirt expects to be able to use
> the Qemu monitor both before and after migration has taken place, on both the
> source and destination nodes.  After migration has taken place is no problem; we
> return to the main qemu select() loop, and we can run monitor commands.
> However, before migration, on the destination side, when we start qemu with a
> command-line like:
>
> qemu-kvm -M pc -S blah blah -incoming tcp://0:4444
>
> we can't run any monitor commands since the migration code is synchronously
> waiting for an incoming tcp connection.  To get around this, the following patch
> adds a new monitor command called "migrate_incoming"; it takes all of the same
> parameters as the command-line option, but just starts it later.  To make sure
> it is safe, you actually have to start with "-incoming monitor"; if you run it
> without that, it will just spit an error at you.  So with this in place, libvirt
> can do the equivalent of:
>
> qemu-kvm -M pc -S blah blah -incoming monitor
>   

I think adding a 'nowait' parameter to migration would make more sense 
than introducing a monitor command.

So:

qemu-kvm -M pc -S blah blah -incoming tcp://0:4444,nowait

 From an implementation perspective, it's just a matter of setting a 
callback for the accept fd I imagine.

Regards,

Anthony Liguori


> (qemu) info cpus
> ...other commands
> (qemu) migrate_incoming tcp://0:4444
> ...wait for migration to start, and then complete
> (qemu) info block
> ...etc.
>
> Signed-off-by: Chris Lalancette <clalance@redhat.com>
>