Re: ARM Snapshots Not Backwards-Compatible

[Andrew Jones]

On Wed, Feb 03, 2021 at 10:45:47AM -0500, Aaron Lindsay wrote:
> On Feb 03 13:44, Andrew Jones wrote:
> > On Wed, Feb 03, 2021 at 10:52:59AM +0000, Peter Maydell 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?
> > 
> > Not even downstream for the Arm architecture, at least not at Red Hat. The
> > support we have for Arm Virt is still limited to the use cases for which
> > it has been deployed. To this day that hasn't included migration[*].
> > 
> > > I think that unless we either (a) start
> > > doing migration-compat testing consistently upstream or
> > 
> > This is the best choice and it can certainly be an additional approach
> > regardless of what goes on downstream. I can look into this. A pointer
> > to the x86 tests would be a good start. It's pretty simple to do a
> > quick migration test between two versions of QEMU, but we need the
> > whole build two versions of QEMU stuff first, which I hope already
> > exists.
> 
> Does this mean that this is largely an issue of developing the tests,
> and not a need for a place to host them? Or would additional
> hardware/hosting be required for these tests to be run on?
>

I wouldn't expect the new tests to require new resources. Whatever
upstream already has for the tests that are already being run should be
sufficient. I think we just need to see if we can develop/enable these
types of tests in the already present CI. At least for starters...

Thanks,
drew