Re: [RFC PATCH 1/7] static_call/x86: Add __static_call_returnl0()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 10, 2020 at 01:56:03AM +0100, Frederic Weisbecker wrote:

> diff --git a/include/linux/static_call.h b/include/linux/static_call.h
> index 695da4c9b338..055544793430 100644
> --- a/include/linux/static_call.h
> +++ b/include/linux/static_call.h
> @@ -136,6 +136,9 @@ extern void arch_static_call_transform(void *site, void *tramp, void *func, bool
>  
>  #ifdef CONFIG_HAVE_STATIC_CALL_INLINE
>  
> +extern int __static_call_return0(void);
> +extern long __static_call_returnl0(void);
> +
>  extern int __init static_call_init(void);
>  
>  struct static_call_mod {
> @@ -187,6 +190,9 @@ extern int static_call_text_reserved(void *start, void *end);
>  
>  #elif defined(CONFIG_HAVE_STATIC_CALL)
>  
> +extern int __static_call_return0(void);
> +extern long __static_call_returnl0(void);
> +
>  static inline int static_call_init(void) { return 0; }
>  
>  struct static_call_key {
> @@ -234,6 +240,9 @@ static inline int static_call_text_reserved(void *start, void *end)
>  
>  #else /* Generic implementation */
>  
> +static inline int __static_call_return0(void) { return 0; }
> +static inline long __static_call_returnl0(void) { return 0; }
> +
>  static inline int static_call_init(void) { return 0; }
>  
>  struct static_call_key {
> diff --git a/kernel/static_call.c b/kernel/static_call.c
> index 84565c2a41b8..3cb371e71be6 100644
> --- a/kernel/static_call.c
> +++ b/kernel/static_call.c
> @@ -438,6 +438,16 @@ int __init static_call_init(void)
>  }
>  early_initcall(static_call_init);
>  
> +int __static_call_return0(void)
> +{
> +	return 0;
> +}
> +
> +long __static_call_returnl0(void)
> +{
> +	return 0;
> +}
> +
>  #ifdef CONFIG_STATIC_CALL_SELFTEST
>  
>  static int func_a(int x)

So yes, we need the out of line copy, but why do we need the int/long
variants?

AFAICT we only need the long version and can cast it to whatever we need
(provided the return value is no bigger than long).