Re: [Xen-devel] Xen-4.5 HVMOP ABI issues

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.12.14 at 15:28, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 04/12/14 13:49, Jan Beulich wrote:
>>>>> On 28.11.14 at 16:46, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 28/11/14 15:18, Jan Beulich wrote:
>>>>>>> On 28.11.14 at 14:55, <andrew.cooper3@xxxxxxxxxx> 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.

Why? We certainly don't guarantee compatibility for undefined
hypercalls to behave in a certain way.

Jan


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