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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 17 Aug 2012, Jan Beulich wrote:
> >>> 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.

The _x variable is still useful to avoid multiple evaluations of hnd,
even though I know that this is not a public header.

What about the following:

/* Cast a XEN_GUEST_HANDLE to XEN_GUEST_HANDLE_PARAM */
#define guest_handle_to_param(hnd, type) ({                \
    typeof((hnd).p) _x = (hnd).p;                          \
    XEN_GUEST_HANDLE_PARAM(type) _y = { _x };              \
    if (&_x != &_y.p) BUG();                               \
    _y;                                                    \
})


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