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

[Eduardo Habkost]

On Mon, Nov 23, 2009 at 09:00:05AM -0600, Anthony Liguori wrote:
<snip>
>
> I think the problem is that you shouldn't be changing the guest visible  
> state in a stable update of qemu.  If you change the guest visible state  
> in a stable update, then you won't be able to support live migration  
> between arbitrary stable versions.  You can't introduce features without  
> introducing forward compatibility issues.  If you're adding new guest  
> visible state, you've added a feature.
>
> This is not a live migration problem, this is a problem with your stable  
> branch policy.

It is not a feature, because the data was already supposed to be part of
the guest visible state, but the implementation was buggy. If the
current implementation were the ultimate authority regarding what is a
bug and what is a new feature, no software would ever had any bug,
everything we call "bug fix" today, would be called "feature".

I would agree with you if the ultimate authority regarding expected
guest visible state was our implementation. But things aren't that
simple: sometimes the definition of "guest visible state" in the
implementation is buggy, and we can't just tell users "the feature your
guests are using shouldn't be part of the guest visible state, don't use
it".

-- 
Eduardo