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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

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.")

 -George

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