Re: [Xen-devel] [RFC] Xen PV Drivers Lifecycle

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 28 Dec 2016, George Dunlap wrote:
> On Tue, Dec 20, 2016 at 11:09 PM, Stefano Stabellini
> <sstabellini@xxxxxxxxxx> wrote:
> > On Tue, 20 Dec 2016, Jan Beulich wrote:
> >> >>> On 20.12.16 at 01:47, <sstabellini@xxxxxxxxxx> wrote:
> >> > ## Design Phase
> >> >
> >> > The first step toward acceptance of a new PV protocol is to write a
> >> > design document and send it to xen-devel. It should cover the xenstore
> >> > handshake mechanism, the ABI, how the protocol works and anything else
> >> > which is required to write an implementation of it. The usage of C-like
> >> > structs to describe language and platform agnostic protocols is
> >> > discouraged.
> >> >
> >> > An attempt should be made for the protocol ABI to be backward compatible
> >> > and OS agnostic, but, realistically, backward and cross-platform
> >> > compatibility are not fully expected at this stage.
> >>
> >> How does backward compatibility matter for a new protocol? Is
> >> this perhaps rather about forward compatibility provisions
> >> (like requiring reserved fields to be zero to allow future use)?
> >
> > By "backward compatibility" I mean promising that, in 5 years time, a
> > new frontend will still be able to connect to a backend written in 2016.
> >
> > This level of support requires an understanding of the protocol and its
> > subtleties which usually only comes from experience. Hence, I am
> > suggesting to make this kind of promises only after the code has lived
> > in-tree for a while and has been subject to wider testing.
> >
> > Maybe I should call it cross-versions compatibility?
> 
> Right, so I think you mean that you design the (new) protocol such
> that *future* versions can be backwards-compatible.
> 
> What if you rephrase it something like the following?
> 
> "An attempt should be made to design the ABI such that it will be OS
> agnostic, that future versions will not need to introduce
> backward-incompatible changes, and so on; but these are not yet hard
> requirements."  (Or perhaps, "not yet first-order goals.")

sounds good to me

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