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

[Gavin Shan <gshan@redhat.com>]

Hi Drew,

On 3/17/21 11:54 PM, Andrew Jones wrote:
> On Wed, Mar 17, 2021 at 11:14:56AM +0000, Peter Maydell wrote:
>> On Wed, 17 Mar 2021 at 10:59, Gavin Shan <gshan@redhat.com> wrote:
>>> 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?
>>
>> Upstream doesn't care about backwards migration. AIUI
>> RedHat as a downstream care about the backwards-migration
>> case in some specific situations, but I don't know if that
>> would include this one.
> 
> Right, we do prefer to be able to support "ping-pong" migrations. For
> example, if we start a virt-5.1 machine on a 5.1 build of QEMU, and then
> migrate it to a 5.2 build of QEMU, we'd like to also be able to go back
> to the 5.1 build.
> 
> I agree this patch is not the right approach. I think the right approach
> is to introduce a compat property and make the "new" section dependent
> on it. And then update the hw_compat_* arrays. Gavin, please take a look
> at "Connecting subsections to properties" of docs/devel/migration.rst.
> 

Agree and thanks for the pointer. I will post another patch to have
something in hw_compat_5_1 to address this particular issue.

> I'm also curious what the state of mach-virt's machine types are for
> migration. It'd be nice to exhaustively test both forward migration of
> all machine types and ping-pong migrations of all machine types. We can
> then consider each issue we find (the pessimist in me suggests we'll find
> more than this pl011 issue) and how/if we want to resolve them.
> 

Yeah, I will think about it and do the testing to see if there are more
issues. Also, it'd better to be integrated to existing testing framework
as you suggested.

Thanks,
Gavin