Re: [Xen-devel] Xen/XenoLinux interface versions

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> On Fri, 2004-03-12 at 00:40, Ian Pratt wrote:
> 
> > Christian made the very sensible suggestion that we should pass
> > the interface version number in to the domain in one of the
> > registers. It could then refuse to run or use a compatibility
> > mode. (hopefully the interface version would be sufficiently
> > close for it to be able to use HYPERVISOR_console_write to tell
> > someone!)
> 
> One of the things I liked about working with L4 (and Linux btw) was that
> the syscall 
> interface was fairly stable and well-documented. Do you think the Xen
> interface will stabilise over time, so that most Xen nodes out there
> will be binary compatible?

That's obviously a desirable aim, but it extends to more than just the
hypercall interface -- for example, the format of I/O buffers and the
shared_info dat astructure.

At the very least I want to avoid changing the interfaces after a
point release 'stabilises' (eg. 1.2 interfaces will not change now,
but 1.3 is still volatile). There is now a hypercall (xen_version())
which returns the currently-runnign point release. This can be used to
implement guest OSes that are portable across stable point releases,
where that matters. 

There are a couple more things which may change the interfaces somewhat
(I/O virtualisation and, to some extent, SMP guest OSes). Once these
are done I'd like to get a 2.0 release out the door with firmer
guarantees regarding interface stability.

Currently each stable point release is providing sufficient extra
functionality that there is strong motivation to upgrade and not look
back. :-)

 -- Keir


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