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Re: ARM Snapshots Not Backwards-Compatible

From: Peter Maydell
Subject: Re: ARM Snapshots Not Backwards-Compatible
Date: Wed, 3 Feb 2021 10:52:59 +0000

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 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".)

-- PMM

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