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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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;
         })

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...

Ian.


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