Re: ARM Snapshots Not Backwards-Compatible

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* 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.

Dave

> thanks
> -- PMM
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK