Re: [PATCH v4 00/37] Clean-ups: qom-ify serial and remove QDEV_PROP_PTR

[Peter Maydell]

On Sun, 1 Dec 2019 at 10:19, Marc-André Lureau
<address@hidden> wrote:
>
> - "serial: register vmsd with DeviceClass"
>
> This is standard qdev-ification, however it breaks backward migration,
> but that's just how qdev_set_legacy_instance_id() works.

I don't understand this part. Surely the whole point
of setting a legacy instance ID is exactly to preserve
migration compatibility? If it doesn't do that then what
does setting legacy ID value do?

thanks
-- PMM