Re: [Qemu-devel] Re: Re: Live migration protocol, device features, ABIs and other beasts

[Paul Brook]

> But it's easy to support migration to old qemu just
> by discarding the INTx state, and this is not
> at all harder, or worse, than migrating from old qemu
> to new one.

Do we really care about migrating to older versions?

Migrating to a new version (backward compatibility) I see the use, it allows 
people to do upgrades with minimal downtime. I have my reservations about how 
feasible this is long-term, but within a release series it's not too bad.

However is migrating to an old version (forward compatibility) really a 
worthwhile thing to support? It sounds like the sort of thing that we're never 
really going to test properly, so will probably fail a good proportion of the 
time anyway. Reading in old state files is a whole lot easier (to write 
maintain, and stay sane) than producing state that is bug-compatible with 
previous versions.
This feels something where the best answer all round is "Don't do that".

Paul