Re: [Qemu-devel] [RFC PATCH v2 00/16] visitor+BER migration format

[Markus Armbruster]

"Dr. David Alan Gilbert" <address@hidden> writes:

> * Eric Blake (address@hidden) wrote:
>> On 04/23/2014 10:37 AM, Dr. David Alan Gilbert (git) wrote:
>> > From: "Dr. David Alan Gilbert" <address@hidden>
>> > 
>> 
>> >    4) At the moment you select BER output format by setting an environment
>> >       variable ( export QEMUMIGFORMAT=BER ) , I need to put more thought
>> >       in to the right way to do this, there are some harder questions like
>> >       what happens to devices that are still using pre-vmstate encodings
>> >       (that are currently sent as blobs) when they eventually convert over
>> >       and thus how to keep compatibility with earlier BER output versions
>> >       where they were blobs.
>> 
>> I don't have good advice on how to address intra-version design (what
>> happens when an old version of BER sends a blob but a new version on the
>> receiving side expects formatted data instead of a blob), other than
>> it's going to be similar to any other intra-version design that we
>> already have to consider when upgrading from old to new qemu.
>> 
>> But for how to select BER format, I _do_ have an idea:
>> 
>> https://lists.gnu.org/archive/html/qemu-devel/2014-04/msg00782.html
>> 
>> Basically, I think that the choice of migration format should be
>> selected via a new extended capability added to
>> migrate-set-capabilities.  Setting the choice at the environment
>> variable is too inflexible (it's locked down for the duration of the
>> entire qemu process), whereas setting it via QMP is desirable (for
>> example, it would let us choose at the time of migration whether we are
>> migrating to an older host and want the old format, or migrating to a
>> file for checkpointing reasons and want the new format).
>
> Yep, that would certainly be easy to do - and I can do that for
> the next version.
> It's more the intra-version I'm worried about, primarily because I don't
> want to have to wait until every device is vmstate'd before moving this
> code forward.
>
> The one thing that the environment variable does make nice and easy,
> for dev, is using it with existing test setups - e.g. running virt-test
> in BER mode or existing mode.

Sounds like a useful hack to speed up development, but not so much like
a useful permanent API :)