Re: Xen-4.5 HVMOP ABI issues

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 04/12/14 13:49, Jan Beulich wrote:
>>>> On 28.11.14 at 16:46, <andrew.cooper3@citrix.com> wrote:
>> On 28/11/14 15:18, Jan Beulich wrote:
>>>>>> On 28.11.14 at 14:55, <andrew.cooper3@citrix.com> wrote:
>>>> The problem is with continuations which reuse the upper bits of the
>>>> input register, not with this HVMOP_op_mask specifically; the
>>>> HVMOP_op_mask simply adds to an existing problem.  This is something
>>>> which needs considering urgently because, as you identify above, we have
>>>> already suffered an accidental ABI breakage with the mem-op widening.
>>> Since we can't retroactively fix the mem-op widening, I still don't see
>>> what's urgent here: As long as we don't change any of these masks,
>>> nothing bad is going to happen. Of course one thing to consider with
>>> this aspect in mind is whether to change the hvm-op or gnttab-op
>>> masks again _before_ 4.5 goes out, based on whether we feel they're
>>> wide enough for the (un)foreseeable future.
>> By urgent, I mean exactly this, while we have the ability to tweak the
>> masks.
> With no-one else voicing an opinion:
>
> For hvmop, the mask currently is 8 bits and we've got 22 ops defined.
>
> For gnttabop, the mask currently is 12 bits and we've got 12 ops defined.
>
> For the latter, we're fine even without further consideration. For the
> former, the two operations actively using the continuation encoding
> are tools-only ones. Since we're fine to alter the tools only interfaces,
> and since it was intended for the tools-only HVM-ops to be split off
> to a separate hypercall (e.g. hvmctl) anyway, the range restriction
> would then no longer be a problem. Plus, in the worst case we could
> always introduce yet another hypercall if we ran out of numbers.

Are you suggesting that we make a new hvmctl now and remove the hvmop
mask before 4.5?  If we ship 4.5 with the hvmop mask, we cannot
subsequently remove it even if all continuable hypercalls move to a
separate hypercall.

>
> Consequently what I'd like to propose (and I guess I'll craft a patch
> as soon as I finished this mail) is that we add comments to these
> masks (also the memop one) to make clear that they mustn't change.
> Additionally for forward compatibility I'd also like to add checks for
> the upper bits to be zero for any of the sub-ops that don't actually
> use them to encode continuation information. Konrad, would you
> consider doing so acceptable for 4.5?

I will have to re-work around this new check for XenServer
compatibility, but I suppose that is fine.  I am certainly in favour of
leaving an ABI comment with the op masks.

~Andrew