Re: [PATCH for-4.14 1/3] xen/arm: atomic: Allow read_atomic() to be used in more cases

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 7 May 2020, Julien Grall wrote:
> Hi,
> 
> On 07/05/2020 21:29, Stefano Stabellini wrote:
> > >   #define read_atomic(p) ({
> > > \
> > > -    typeof(*p) __x;                                                     \
> > > -    switch ( sizeof(*p) ) {                                             \
> > > -    case 1: __x = (typeof(*p))read_u8_atomic((uint8_t *)p); break;      \
> > > -    case 2: __x = (typeof(*p))read_u16_atomic((uint16_t *)p); break;    \
> > > -    case 4: __x = (typeof(*p))read_u32_atomic((uint32_t *)p); break;    \
> > > -    case 8: __x = (typeof(*p))read_u64_atomic((uint64_t *)p); break;    \
> > > -    default: __x = 0; __bad_atomic_size(); break;                       \
> > > -    }                                                                   \
> > > -    __x;                                                                \
> > > +    union { typeof(*p) val; char c[0]; } x_;                            \
> > > +    read_atomic_size(p, x_.c, sizeof(*p));                              \
> > 
> > Wouldn't it be better to pass x_ as follows:
> > 
> >      read_atomic_size(p, &x_, sizeof(*p));
> 
> I am not sure to understand this. Are you suggesting to pass a pointer to the
> union?

Yes. Would it cause a problem that I couldn't spot?