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

[Gavin Shan <gshan@redhat.com>]

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