Re: [Qemu-devel] [PATCH 1/1] s390x/migration: Introduce 2.4 machine

[Juan Quintela <quintela@redhat.com>]

Christian Borntraeger <borntraeger@de.ibm.com> wrote:

First of all

Reviewed-by: Juan Quintela <quintela@redhat.com>

For the patch.

But one said that, I don't agree with the commint text.


> The section footer changes commit f68945d42bab ("Add a protective
> section footer") and commit 37fb569c0198 ("Disable section footers
> on older machine types") broke migration for any non-versioned
> machines.

If broke migration for 2.4 -> 2.3 for machines that don't care about
compatibility.  If they care, they are versioned O:-)  Right now ppc &
x86.  I guess that s390 and arm will follow in due curse.

> While one can argue that section footer should be enabled
> explicitely for new versions instead of disabled for old ones,
> this pinpoints to a problem of s390-ccw-machines: it needs to
> be versioned to be compatible with future changes in common
> code data structures such as section footers.

It is done explicitely the other way around.  New way is the default
way.  If we did it in a special way in the past, we add a switch.  If at
some point we remove the old machine type for any reason, we don't have
to change anything on $latest.

Basically the idea here is that (until now) only x86 cared about
backwards compatibility, so when a migration changed happened because it
was good for any reason, normal devices do changes, and then x86 try to
fix the pieces after the fact.  That is going to continue, just that now
more architectures care, and then we should detect this kind of problems
much earlier.

> Let's introduce a version scheme for s390-ccw-virtio machines.
> We will use the old s390-ccw-virtio name as alias to the latest
> version as all existing libvirt XML for the ccw type were expanded
> by libvirt to that name.
>
> The only downside of this patch is, that the old alias s390-ccw
> will no longer be available as machines can have only one alias,
> but it should not really matter.

Should we change to a list?

Later, Juan.