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

[Marc-André Lureau]

Hi

On Sun, Dec 1, 2019 at 9:18 PM Peter Maydell <address@hidden> wrote:
>
> 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?
>

It works in old->new direction only, because new code can match the
legacy instance id.

But when going from new->old, the legacy instance id is lost, as it
uses new 0-based instance_id.

This is just the way we have done so far, but I wish to be corrected
if I am wrong.

See the commit description for a bit more details.


-- 
Marc-André Lureau