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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.07.2024 14:00, Roger Pau Monné wrote:
> 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?

Only if we stick to using unsigned long for a ## n ## _. Afaics there's
nothing wrong with making that void *, too.

Jan