Re: [Xen-devel] [PATCH 4/4] xen/public: arm: rework the macro set_xen_guest_handle_raw

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.11.15 at 17:22, <Ian.Jackson@xxxxxxxxxxxxx> wrote:
> Julien Grall writes ("Re: [PATCH 4/4] xen/public: arm: rework the macro 
> set_xen_guest_handle_raw"):
>> On 03/11/15 14:18, Stefano Stabellini wrote:
>> >> +#define set_xen_guest_handle_raw(hnd, val)                              \
>> >> +    do {                                                                \
>> >> +        /* Check if the handle is 64-bit (i.e 8-byte) */                \
>> >> +        (void) sizeof(struct { int : -!!(sizeof (hnd) != 8); });        \
>> >> +        /* Check if the type of val is compatible with the handle */    \
>> >> +        (void) sizeof((val) != (hnd).p);                                \
>> >> +        (hnd).q = (uint64_t)(uintptr_t)(val);                           \
>> >>      } while ( 0 )
>> >>  #define set_xen_guest_handle(hnd, val) set_xen_guest_handle_raw(hnd, val)
> 
> I hate to throw yet more variables into this, but I had a discussion
> with some C experts in the pub about this problem.  Further thought
> (including a red herring where someone suggested that memcpy would
> "launder" the types) results in this proposal:
> 
> 
> union xen_guest_handle_TYPE {
>   TYPE *p;
>   uint64_t align;
> };
> 
> struct xen_guest_handle__pointer {
>   uint8_t p0,p1,p2,p3;
> };
> 
> /*
>  * void set_xen_guest_handle_raw(xen_guest_handle_TYPE *hnd, TYPE *val);
>  */
> #define set_xen_guest_handle_raw(hnd, val)
>   do {
>      void *copy = (val);
>      struct xen_guest_handle__pointer *src = &(hnd);
>      struct xen_guest_handle__pointer *dst = &(xen_xgh_copy);
>      dst->p0 = src->p0;
>      dst->p1 = src->p1;
>      dst->p2 = src->p2;
>      dst->p3 = src->p3;
>      dst[1]->p0 = 0;
>      dst[1]->p1 = 0;
>      dst[1]->p2 = 0;
>      dst[1]->p3 = 0;
>      sizeof((hnd).p == (val)); /* typecheck */
>   }
> 
> #define get_xen_guest_handle(hnd) ((hnd).p)
> 
> 
> The above is legal for the following reasons (C99 6.5):
> 
> Considering the accesses to src and dst.  These are legal according to
> (7) point 5.  (Accesses via a character type are always legal.)
> 
> After the above macro has been used to set, what is the effective type
> of hnd for a subsequent get_xen_guest_handle ?

All quite interesting, but pretty moot with there not being any
get_xen_guest_handle() anymore.

Jan


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