Re: [PATCH] hw/char/pl011: Fix clock migration failure

[Gavin Shan]

Hi Peter,

On 3/17/21 9:40 PM, Peter Maydell wrote:
> On Wed, 17 Mar 2021 at 10:37, Gavin Shan <gshan@redhat.com> wrote:
> > On 3/17/21 8:09 PM, Peter Maydell wrote:
> > > On Wed, 17 Mar 2021 at 04:44, Gavin Shan <gshan@redhat.com> wrote:
> > > >    static const VMStateDescription vmstate_pl011 = {
> > > >        .name = "pl011",
> > > >        .version_id = 2,
> > > >        .minimum_version_id = 2,
> > > > +    .post_load = pl011_post_load,
> > > >        .fields = (VMStateField[]) {
> > > >            VMSTATE_UINT32(readbuff, PL011State),
> > > >            VMSTATE_UINT32(flags, PL011State),
> > > > @@ -355,10 +355,6 @@ static const VMStateDescription vmstate_pl011 = {
> > > >            VMSTATE_INT32(read_trigger, PL011State),
> > > >            VMSTATE_END_OF_LIST()
> > > >        },
> > > > -    .subsections = (const VMStateDescription * []) {
> > > > -        &vmstate_pl011_clock,
> > > > -        NULL
> > > > -    }
> > > >    };
> > >
> > > Doesn't dropping the subsection break migration compat ?
> >
> > It's why this patch needs to be backported to stable branches.
> > In that way, we won't have migration compatible issue.
>
> No, migration has to work from the existing already
> shipped 5.1, 5.2, etc releases to 6.0 (assuming you use
> the correct "virt-5.2" &c versioned machine type.)

Commit aac63e0e6ea3 ("hw/char/pl011: add a clock input") is merged
to v5.2.0. The migration failure happens during migration from v6.0
to v5.1 with machine type as "virt-5.1", instead of migrating from
v5.1 to v6.0. One question is if we need support backwards migration?

If we do support backwards migration, I think there are two options:
(1) merge this patch and backport it to v5.2+; (2) Backport commit
aac63e0e6ea3 to v5.2-. I guess (1) would be right way to go because
it's less effort than (2). Besides, the clock needn't to be migrated
if I'm correct.

Thanks,
Gavin