Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

Rasmus Villemoes
2018-11-30T22:16:40Z

On 29/11/2018 20.22, Josh Poimboeuf wrote:
> On Thu, Nov 29, 2018 at 02:16:48PM -0500, Steven Rostedt wrote:
>>> and honestly, the way "static_call()" works now, can you guarantee
>>> that the call-site doesn't end up doing that, and calling the
>>> trampoline function for two different static calls from one indirect
>>> call?
>>>
>>> See what I'm talking about? Saying "callers are wrapped in macros"
>>> doesn't actually protect you from the compiler doing things like that.
>>>
>>> In contrast, if the call was wrapped in an inline asm, we'd *know* the
>>> compiler couldn't turn a "call wrapper(%rip)" into anything else.
>>
>> But then we need to implement all numbers of parameters.
>
> I actually have an old unfinished patch which (ab)used C macros to
> detect the number of parameters and then setup the asm constraints
> accordingly. At the time, the goal was to optimize the BUG code.
>
> I had wanted to avoid this kind of approach for static calls, because
> "ugh", but now it's starting to look much more appealing.
>
> Behold:
>
> diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
> index aa6b2023d8f8..d63e9240da77 100644
> --- a/arch/x86/include/asm/bug.h
> +++ b/arch/x86/include/asm/bug.h
> @@ -32,10 +32,59 @@
>
> #ifdef CONFIG_DEBUG_BUGVERBOSE
>
> -#define _BUG_FLAGS(ins, flags) \
> +#define __BUG_ARGS_0(ins, ...) \
> +({\
> + asm volatile("1:\t" ins "\n"); \
> +})
> +#define __BUG_ARGS_1(ins, ...) \
> +({\
> + asm volatile("1:\t" ins "\n" \
> + : : "D" (ARG1(__VA_ARGS__))); \
> +})
> +#define __BUG_ARGS_2(ins, ...) \
> +({\
> + asm volatile("1:\t" ins "\n" \
> + : : "D" (ARG1(__VA_ARGS__)), \
> + "S" (ARG2(__VA_ARGS__))); \
> +})
> +#define __BUG_ARGS_3(ins, ...) \
> +({\
> + asm volatile("1:\t" ins "\n" \
> + : : "D" (ARG1(__VA_ARGS__)), \
> + "S" (ARG2(__VA_ARGS__)), \
> + "d" (ARG3(__VA_ARGS__))); \
> +})

wouldn't you need to tie all these to (unused) outputs as well as adding
the remaining caller-saved registers to the clobber list? Maybe not for
the WARN machinery(?), but at least for stuff that should look like a
normal call to gcc? Then there's %rax which is either a clobber or an
output, and if there's not to be a separate static_call_void(), one
would need to do some __builtin_choose_expr(__same_type(void, f(...)), ...).

Rasmus