Re: [PATCH v3 07/33] serial: register vmsd with DeviceClass

[Marc-André Lureau]

On Tue, Nov 19, 2019 at 2:35 PM Peter Maydell <address@hidden> wrote:
>
> On Tue, 19 Nov 2019 at 10:23, Marc-André Lureau
> <address@hidden> wrote:
> > On Mon, Nov 18, 2019 at 6:22 PM Peter Maydell <address@hidden> wrote:
> > > Did you test whether migration still works from a QEMU
> > > version without this patch to one with it? (The migration
> >
> > Yes, I thought I did test correctly, but I realized testing with x86
> > isn't correct.
> >
> > So with arm/musicpal for ex, I can migrate from before->after, however
> > after->before won't work. Is that ok?
>
> Broadly speaking, the only case where we care about not
> breaking cross-version migration is where we have a versioned
> machine type. So musicpal doesn't matter too much. Beyond
> that, yes, generally before->after is more important than
> after->before. I have a feeling Red Hat downstream cares about
> after->before migration at least for x86 but you or your colleagues
> would know that better than me :-)
>
> > > vmstate code is too complicated for me to be able to figure
> > > out whether passing the 'dev' pointer makes a difference
> > > to whot it names the state sections and whether the
> > > 'qdev_set_legacy_instance_id' suffices to avoid problems.)
> >
> > I don't see a way to fix after->before, because the instance id is
> > initially 0 with the new code, and the old code expect a different
> > value.
>
> Can you explain how the instance ID stuff works? I was
> expecting that the result of setting the legacy instance ID
> would just be that the new version would always have
> the older setting, so if it works for old->new it would also
> work for new->old. But as I say I don't understand this bit
> of the migration code.

>From what I understand, the alias_id is only used in
savevm.c:find_se(), and thus can only be used to match against
"legacy" instance id values. On new code, instance_id is generated
incrementally from 0 with calculate_new_instance_id(), based on
"qdev-path/vmsd-name".



-- 
Marc-André Lureau