Re: [Xen-devel] [PATCH v9 10/11] common: add a new mappable resource type: XENMEM_resource_grant_table

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.10.17 at 10:54, <Paul.Durrant@xxxxxxxxxx> wrote:
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: 11 October 2017 09:47
>> >>> On 10.10.17 at 18:01, <Paul.Durrant@xxxxxxxxxx> wrote:
>> >> > @@ -993,6 +1018,11 @@ static int acquire_resource(const
>> >> xen_mem_acquire_resource_t *xmar)
>> >> >                                           xmar->nr_frames, mfn_list);
>> >> >          break;
>> >> >
>> >> > +    case XENMEM_resource_grant_table:
>> >> > +        rc = acquire_grant_table(d, xmar->id, xmar->frame,
>> >> > +                                 xmar->nr_frames, mfn_list);
>> >> > +        break;
>> >>
>> >> Is this really generally useful with mfn_list[] having just two entries?
>> >>
>> >
>> > Good point. I'll increase the size of the array in this patch (to the
>> > default table size of 32... I think that's a reasonable value to choose).
>> 
>> I suppose for the only current use you have for this (seeding the
>> grant table from the tool stack) even the two entries you have
>> right now would suffice. If, however, a full grant table is supposed
>> to be accessible this way, I can't see how a static upper limit will do.
>> Or if you intend the caller to do multiple invocations in such a case,
>> there ought to be a way to find out the (implementation) limit.
> 
> I'm open to ideas but there clearly needs to be some sort of upper limit, or 
> we do away with being able to map multiple frames in a single invocation. The 
> dm_op hypercalls currently have a similar upper limit on the size of the 
> buffer array. I'd rather not have to introduce another hypercall just to find 
> out such a thing. It's a tools-only hypercall so could I not just add a 
> comment on what the limit currently is?

Hmm, that would be an option, but I'd prefer if we could get away
without. And no, I wasn't suggesting to introduce yet another
hypercall. Instead how about the handle being a null one asking
for the implementation limit to be returned in nr_frames (or, to
keep that IN only, in the re-purposed pad field)?

Jan


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