Re: ARM Snapshots Not Backwards-Compatible

[Philippe Mathieu-Daudé]

On 2/3/21 11:59 AM, Dr. David Alan Gilbert wrote:
> * Peter Maydell (peter.maydell@linaro.org) wrote:
>> On Wed, 3 Feb 2021 at 10:49, Dr. David Alan Gilbert <dgilbert@redhat.com> 
>> wrote:
>>>
>>> * Peter Maydell (peter.maydell@linaro.org) wrote:
>>>> On Wed, 3 Feb 2021 at 10:28, Dr. David Alan Gilbert <dgilbert@redhat.com> 
>>>> wrote:
>>>>>
>>>>> * Philippe Mathieu-Daudé (philmd@redhat.com) wrote:
>>>>>> Cc'ing migration team and qemu-arm@ list.
>>>>>
>>>>> I'll have to leave the detail of that to the ARM peole; but from a
>>>>> migration point of view I think we do want the 64 bit ARM migrations to
>>>>> be stable now.  Please tie incompatible changes to machine types.
>>>>
>>>> That is the intention, but because there's no upstream testing
>>>> of migration compat, we never notice if we get it wrong.
>>>> What is x86 doing to keep cross-version migration working ?
>>>
>>> I know there used to be some of our team running Avocado tests for
>>> compatibility regularly, I'm not sure of the current status.
>>> It's something we also do regularly around when we do downstream
>>> releases, so we tend to catch them then, although even on x86 that
>>> often turns out to be a bit late.
>>
>> So downstream testing only?
> 
> I thought there used to be some regular avocado testing of upstream but
> I'm not sure if it's all architectures and I'm not sure if it's still
> happening; I haven't seen any migration issues from it for a while,
> which makes me think it isn't.
> 
>> I think that unless we either (a) start
>> doing migration-compat testing consistently upstream or (b) RedHat or
>> some other downstream start testing and reporting compat issues
>> to us for aarch64 as they do for x86-64, in practice we're just
>> not going to have working migration compat despite our best
>> intentions. (None of the issues Aaron raises were deliberate
>> compat breaks -- they're all "we made a change we didn't think
>> affected migration but it turns out that it does".)
> 
> I'd agree; we still hit this too often on x86 as well.

Cc'ing avocado-devel, since previously discussed:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg654139.html