Re: [Qemu-devel] [PATCH] migration: Correctly handle subsections with no 'needed' function

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> On 7 August 2018 at 16:05, Juan Quintela <quintela@redhat.com> wrote:
> > Peter Maydell <peter.maydell@linaro.org> wrote:
> >> So it's better to use a needed-always subsection, which will give
> >> the desired behaviour:
> >>  * new QEMU -> new QEMU: state is always migrated
> >>  * old QEMU -> new QEMU: migration doesn't fail, and guest
> >>    will work assuming it didn't care about this corner of the
> >>    device's functionality (the device will end up with state
> >>    as it was at reset, or possibly special-cased via
> >>    pre_load/post_load hooks for the "section not present" case)
> >
> > we break by definiton new QEMU -M <old machine type> into old QEMU.
> 
> Is that supposed to work? I always thought that we never
> supported migration back to an older QEMU version like that.

Downstream we do a lot of hard work to keep that working for versioned 
machine types;  using subsections makes this a lot easier and if it's
done right in the first place then it works upstream as well.

> In any case, for all the devices here the machine types are
> not versioned.

Right.

Dave

> thanks
> -- PMM
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK