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 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".

Yes, you are right. I'll use a temporary pointer.

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