Re: [PATCH v2 3/3] pci: ROM preallocation for incoming migration

[Juan Quintela <quintela@redhat.com>]

"Michael S. Tsirkin" <mst@redhat.com> wrote:

>> > CC pbonzini,dgilbert,quintela,armbru : guys, is poking at runstate_check like
>> > this the right way to figure out we are not going to use the
>> > device locally before incoming migration will overwrite ROM contents?
>> 
>> RUN_STATE_INMIGRATE is set in the only one place in qemu_init() when
>> we parse cmdline option -incoming. VM is not running for sure. And
>> starting the VM comes with changing the state. So it's OK.
>> 
>> The possible problem, if we add netcard on target which we didn't
>> have on source. I now checked, this works.. But that doesn't seem
>> correct to add device that was not present on source - how would it
>> work - it's not guaranteed anyway.
>
> You can add it on source too while migration is in progress, no?

DeviceState *qdev_device_add_from_qdict(const QDict *opts,
                                        bool from_json, Error **errp)
{
    ....
    if (!migration_is_idle()) {
        error_setg(errp, "device_add not allowed while migrating");
        return NULL;
    }

It should be similar for unplug.

We only support hotplug for some devices during migration, and we
shouldn't need any.

What I think he means is that you can add a device on the command line
on destination that don't exist on the source machine, and that will
confuse things.

In that case, I would say that the problem is that you are doing
something not supported.  You are expected that when you run migration
you use the same command line that on source, module whatever
hot[un]plug operations you have done before migration.

Anything else is not supported.
And for instance, if you are using libvirt, it will do the right thing.

Later, Juan.