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 15:47, Stefano Stabellini 
> >>> <stefano.stabellini@xxxxxxxxxxxxx>
> wrote:
> > 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.
> 
> But we had settled on returning hnd unmodified when both
> handle types are the same.
> 
> > 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;                                                    \
> > })
> 
> Since this is not a public header, something like this (untested,
> so may not compile as is)
> 
> #define guest_handle_to_param(hnd, type) ({                \
>     (void)(typeof((hnd).p)0 == (XEN_GUEST_HANDLE_PARAM(type){}).p); \
>     (hnd);                                                    \
> })
> 
> is what I was thinking of.
> 

this is how it would look like:

#define guest_handle_to_param(hnd, type) ({                  \
    /* type checking: make sure that the pointers inside     \
     * XEN_GUEST_HANDLE and XEN_GUEST_HANDLE_PARAM are of    \
     * the same type, than return hnd */                     \
    (void)((typeof(&(hnd).p)) 0 ==                           \
        (typeof(&((XEN_GUEST_HANDLE_PARAM(type)) {}).p)) 0); \
    (hnd);                                                   \
})


Honestly I have very rarely seen anything less readable, but at least is
very compact.
For ARM I was going to go with the following, that is only slightly more
readable:

#define guest_handle_to_param(hnd, type) ({                  \
    typeof((hnd).p) _x = (hnd).p;                            \
    XEN_GUEST_HANDLE_PARAM(type) _y = { _x };                \
    /* type checking: make sure that the pointers inside     \
     * XEN_GUEST_HANDLE and XEN_GUEST_HANDLE_PARAM are of    \
     * the same type, than return hnd */                     \
    (void)(&_x == &_y.p);                                    \
    _y;                                                      \
})

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