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

[Michael S. Tsirkin]

On Sun, Nov 22, 2009 at 09:49:26AM -0600, Anthony Liguori wrote:
>>    We cannot even create a new 'hack section' for new code since the
>>    sections are ordered and expected to be exact match on the
>>    destination.
>>
>>    The result is that new->old migration cannot work. This is not cross
>>    releases even! It means that even a small bug in current release
>>    prevents live migration between various instances of the code.
>>    It forces us to decide whether to fix pvclock migration issue vs
>>    allow new->old migration. Another ugly hack is to add cmdline that
>>    will control this behavior. Still it's a pain to mgmt stack and
>>    users.
>
> This is a pretty normal policy (backwards compat but not forwards compat).

No one is asking that old qemu magically understands new format. It
would be enough that new qemu is able to migrate to a format that old
one understands.

This is backwards compatibility, not forwards compatibility.

If a new version of a word
processor won't save in a format that old version
can read, is it still backwards compatible?

-- 
MST