Re: [PATCH v2] x86/altcall: further refine clang workaround

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jul 29, 2024 at 12:47:09PM +0200, Jan Beulich wrote:
> On 29.07.2024 12:30, Roger Pau Monne wrote:
> > --- a/xen/arch/x86/include/asm/alternative.h
> > +++ b/xen/arch/x86/include/asm/alternative.h
> > @@ -183,13 +183,13 @@ extern void alternative_branches(void);
> >   * https://github.com/llvm/llvm-project/issues/12579
> >   * https://github.com/llvm/llvm-project/issues/82598
> >   */
> > -#define ALT_CALL_ARG(arg, n)                                            \
> > -    register union {                                                    \
> > -        typeof(arg) e[sizeof(long) / sizeof(arg)];                      \
> > -        unsigned long r;                                                \
> > -    } a ## n ## _ asm ( ALT_CALL_arg ## n ) = {                         \
> > -        .e[0] = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); })\
> > -    }
> > +#define ALT_CALL_ARG(arg, n)                                             \
> > +     register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) = ({   \
> > +         unsigned long tmp = 0;                                          \
> > +         *(typeof(arg) *)&tmp = (arg);                                   \
> > +         BUILD_BUG_ON(sizeof(arg) > sizeof(void *));                     \
> 
> With this, even more so than before, I think the type of tmp would better
> be void * (or the BUILD_BUG_ON() be made use unsigned long, yet I consider
> that less desirable).

Won't using void * be uglier, as we then need to cast the last tmp
statement to unsigned long?

> As a nit, I also don't think the backslashes need
> moving out by one position. Finally I'm afraid you're leaving stale the
> comment ahead of this construct.

Yes, thanks for noticing.  Will attempt to adjust the comment.

Roger.