Re: [Xen-devel] [PATCH v3 4/6] xen: introduce XEN_GUEST_HANDLE_PARAM

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.08.12 at 10:02, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Thu, 2012-08-16 at 18:10 +0100, Stefano Stabellini wrote:
>> On Thu, 16 Aug 2012, Jan Beulich wrote:
>> > >>> On 16.08.12 at 17:54, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>> > > Seeing the patch I btw realized that there's no easy way to
>> > > avoid having the type as a second argument in the conversion
>> > > macros. Nevertheless I still don't like the explicitly specified type
>> > > there.
>> > 
>> > Btw - on the architecture(s) where the two handles are identical
>> > I would prefer you to make the conversion functions trivial (and
>> > thus avoid making use of the "type" parameter), thus allowing
>> > the type checking to occur that you currently circumvent.
>> 
>> OK, I can do that.
> 
> Will this result in the type parameter potentially becoming stale?
> 
> Adding a redundant pointer compare is a good way to get the compiler to
> catch this. Smth like;
> 
>         /* Cast a XEN_GUEST_HANDLE_PARAM to XEN_GUEST_HANDLE */
>         #define guest_handle_from_param(hnd, type) ({
>             typeof((hnd).p) _x = (hnd).p;
>             XEN_GUEST_HANDLE(type) _y;
>             &_y == &_x;
>             hnd;
>          })

Ah yes, that's a good suggestion.

> I'm not sure which two pointers of members of the various structs need
> to be compared, maybe it's actually &_y.p and &hnd.p, but you get the
> idea...

Right, comparing (hnd).p with _y.p would be the right thing; no
need for _x, but some other (mechanical) adjustments would be
necessary.

Jan


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