Linux Kernel should I use asmlinkage for a function that implements a system call?

Question

I am trying to implement a new syscall in linux kernel, so I wrote:

asmlinkage int my_func(void) {
    return my_func_internal();
}

my question, should I define my_func_internal as asmlinkage or not?

in other words, should I write A or B?

A) asmlinkage int my_func_internal(void) {return 1;}

B) int my_func_internal(void) {return 1;}

I would like some explanation too

Note: I have added my_func to syscalls.h should I add the internal one too (probably the answer is no)

Answer 1

It doesn't matter (for correctness) what calling convention you use for functions that aren't called directly by hand-written asm. (Which syscall implementation functions might be on some architectures, that's why they should be asmlinkage .) As long as all callers can see a prototype that matches the definition, it will work.

If asmlinkinkage is a different calling convention from the default one (eg on i386, asmlinkage means to use stack args, overriding the -mregparm=3 build option that makes internal functions use register args), the compiler will have to emit a definition for my_func that handles the difference if it calls a function that isn't asmlinkage . Or simply inline my_func_internal() into it.

If they use the same calling convention, and the compiler chooses not to inline, it could just do an optimized tailcall to my_func_internal , eg on x86 jmp my_func_internal . So there's a possible efficiency advantage to using the same calling convention if there's a possibility of an optimized tailcall. Otherwise don't; asmlinkage makes the calling convention less efficient on i386.

(IIRC, asmlinkage has no effect on x86-64 and most other modern ISAs with register-args calling conventions; the default calling convention on x86 is already good so the kernel doesn't need to override it with -mregparm=3 like it does on i386.)

In your example where there are no args, there's no difference.

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BTW, the usual naming convention for the function name is sys_foo to implement a system-call called foo . ie the function that will get called when user-space passes __NR_foo as the call number.

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Note: I have added my_func to syscalls.h should I add the internal one too (probably the answer is no)

Of course not, unless my_func_internal implements a different system call that you want user-space to be able to call directly.