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

[Jan Beulich <jbeulich@xxxxxxxx>]

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). 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.

Jan