Re: [Qemu-devel] [PATCH 14/15] hw/block/virtio-blk: create num_queues vqs if dataplane is enabled

[Paolo Bonzini]

Il 01/08/2014 09:35, Ming Lei ha scritto:
> OK, I will convert non-dataplane to support multi virtqueues in V1,
> and the conversion is not difficult and straightforward.
> 
> BTW, docs/migration.txt mentions that "QEMU has to be launched
> with the same arguments the two times", so can I understand that
> migration from dataplane to non-dataplane shouldn't have been
> allowed?

You're right, that text gives a rule of thumb but it is not absolutely true.

It is for example obviously okay to have different paths for drives in
the two invocations.  It is also okay to use different formats (raw vs.
qcow2) if, for example, you are migrating disk contents (either with
"migrate -b" or with the embedded NBD server).

Dataplane is another such case where an option does not affect how the
device behaves.  It is a bit more clearer if you think of it as just
"use an iothread different from the default one", which is what we're
aiming at.  Everything else is working around limitations in QEMU, and
will disappear once these limitations are lifted.

Paolo