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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.08.12 at 19:08, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx> 
>>> wrote:
> On Thu, 16 Aug 2012, Jan Beulich wrote:
>> >>> On 16.08.12 at 16:50, Stefano Stabellini 
>> >>> <stefano.stabellini@xxxxxxxxxxxxx>  wrote:
>> > +#define set_xen_guest_handle_raw(hnd, val)                  \
>> > +    do { if ( sizeof(hnd) == 8 ) *(uint64_t *)&(hnd) = 0;   \
>> 
>> If you made the "normal" handle a union too, you could avoid
>> the explicit cast (which e.g. gcc, when not passed
>> -fno-strict-aliasing, will choke on) and instead use (hnd).q (and
>> at once avoid the double initialization of the low half).
>> 
>> Also, the condition to do this could be "sizeof(hnd) > sizeof((hnd).p)",
>> usable at once for 64-bit avoiding a full double initialization there.
>> 
>> > +         (hnd).p = val;                                     \
>> 
>> In a public header you certainly want to avoid evaluating a
>> macro argument twice.
> 
> That's a really good suggestion.
> I am going to make both handles unions and therefore
> set_xen_guest_handle_raw becomes:
> 
> #define set_xen_guest_handle_raw(hnd, val)                  \
>     do { (hnd).q = 0;                                       \
>          (hnd).p = val;                                     \
>     } while ( 0 )

But that still doesn't eliminate the double evaluation of "hnd".

Jan


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