Re: [PATCH v2 4/4] virtio-net: Add support for USO features

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Aug 02, 2024 at 12:26:22PM -0400, Peter Xu wrote:
> And that's why I was thinking (where I totally agree with you on this) that
> whether we should settle a short term plan first to be on the safe side
> that we start with migration always being compatible, then we figure the
> other approach.


We have two big issues around migration compatibility we never solved:

- some guest visible behaviour depends on a package outside of qemu:
  as that package can change, so can qemu behaviour

- sometimes we change guest visible behaviour and only
  discover this after the release: fixing that breaks
  migration to one version, not fixing breaks migration to another


These, to me, look similar enough that I feel we should look
at them together from QAPI POV.

Both issues sometimes can have work-arounds, enabling these
would be nice.
Also, both issues have a clean solution, which can come in
two flavors:
1. basic: detecting incompatibility
and not starting qemu on destination (or failing migration,
possibly early, which I consider a less clean solution).
2. advanced: ability to go from a set of configurations to
a flag making them compatible.


-- 
MST