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

[Tim Deegan <tim@xxxxxxx>]

At 14:28 +0000 on 04 Dec (1417699730), Andrew Cooper 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.

I think we can if the only hypercalls that use continuations are
tools-only (and so not liable to work across migration anyway).

Tim.

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