Re: [Qemu-devel] [PATCHv2] rtl8139: save/load RxMulOk counter (again)

[Paolo Bonzini]

On 21/06/2016 11:36, David Vrabel wrote:
> On 21/06/16 08:35, Paolo Bonzini wrote:
>>
>>
>> On 21/06/2016 03:44, Jason Wang wrote:
>>>
>>>
>>> On 2016年06月21日 01:53, David Vrabel wrote:
>>>> Commit 9d29cdeaaca3a0383af764000b71492c4fc67c6e (rtl8139: port
>>>> TallyCounters to vmstate) introduced in incompatibility in the v4
>>>> format as it omitted the RxOkMul counter.
>>>>
>>>> There are presumably no users that were impacted by the v4 to v4'
>>>> breakage, so increase the save version to 5 and re-add the field,
>>>> keeping backward compatibility with v4'.
>>>>
>>>> We can't have a field conditional on the section version in
>>>> vmstate_tally_counters since this version checked would not be the
>>>> section version (but the version defined in this structure).  So, move
>>>> all the fields into the main state structure.
>>>>
>>>> Signed-off-by: David Vrabel <address@hidden>
>>>
>>> Migration to old version is important for the user and this patch seems
>>> to break this. How about something like:
>>>
>>> - introduce a subsection for RXOKMul
>>> - only migrate it for new version (e.g >= 2.7)
> 
> I don't see how this can work with snapshots where the QEMU version that
> is going to restore the snapshot is not known in advance.

By "new version" he meant the versioned machine types, e.g.
pc-i440fx-2.6 and older wouldn't migrate it.

>> Introducing a subsection is not really necessary if the value is going
>> to be migrated always, and upstream generally does not have "migrate it
>> only in some version" checks.  This is left for downstreams to implement
>> if they care.  We just don't have the manpower to ensure that migration
>> to older versions works between all releases of QEMU.
> 
> So is this patch acceptable as is?

I think it is.

Paolo