What is asmlinkage ?

[Rajat Sharma]

> asmlinkage is defined for almost all arch:
> grep asmlinkage arch/arm/*/* and u got the answer.

I didn't see a definition of macro atleast in linux source I was browsing
(3.2.0), Could you please point out to any one you have found.

Alternatively I tried more precise search:
linux-source-3.2.0$ grep -R "#define asmlinkage" arch/arm/

No output. Although similar search gave following result for x86:

linux-source-3.2.0$ grep -R "#define asmlinkage" arch/x86/
arch/x86/include/asm/linkage.h:#define asmlinkage CPP_ASMLINKAGE
__attribute__((regparm(0)))
arch/x86/include/asm/linkage.h:#define asmlinkage_protect(n, ret, args...) \

Typically it should be defined in arch specific linkage.h file only, for
arm there was none I found. If none is found, default definition is given
from inlucde/linux/linkage.h which is nop:

#ifndef asmlinkage
#define asmlinkage CPP_ASMLINKAGE
#endif

CPP_ASMLINKAGE is just extern C stuff:

#ifdef __cplusplus
#define CPP_ASMLINKAGE extern "C"
#else
#define CPP_ASMLINKAGE
#endif

Indeed the only meaningful definition I found for x86_32 only in
arch/x86/include/asm/linkage.h:

#ifdef CONFIG_X86_32
#define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0)))

regparm controls number or integer arguments passed to a function. And it
is specific to x86 only according to gcc manuals.

Hence the above wiki FAQ seems valid for x86_32 arch, although it should
specifically mention this arch dependence to avoid confusion on public
forums.

-Rajat


On Fri, Jan 11, 2013 at 8:43 AM, Peter Teoh <htmldeveloper at gmail.com> wrote:

> asmlinkage is defined for almost all arch:
>
> grep asmlinkage arch/arm/*/* and u got the answer.
>
> It seemed that:
>
>  http://kernelnewbies.org/FAQ/asmlinkage
>
> gave the impression that asmlinkage is only for system call, or associated
> with it.   Not really, nothing to do with system call actually.
>
> Even though majority (or ALL) of syscall are defined with asmlinkage, but
> there are many that are not, eg, in arch/arm subdirectory:
>
> kernel/irq.c:asmlinkage void __exception_irq_entry
> kernel/process.c:asmlinkage void ret_from_fork(void)
> __asm__("ret_from_fork");
> kernel/smp.c:asmlinkage void __cpuinit secondary_start_kernel(void)
> kernel/smp.c:asmlinkage void __exception_irq_entry do_IPI(int ipinr,
> struct pt_regs *regs)
>
> just a few examples.   Essentially, it is just declared so that the name,
> for example, "do_IPI" can be called from assembly, for example in the
> following pair (arch/arm assumed):
>
> /kernel/smp.c:
> asmlinkage void __exception_irq_entry do_IPI(int ipinr, struct pt_regs
> *regs)
>
> ./include/asm/entry-macro-multi.S:
> bne do_IPI
>
> More info:
>
>
> http://stackoverflow.com/questions/10060168/is-asmlinkage-required-for-a-c-function-to-be-called-from-assembly
>
> From above, asmlinkage is also NOT the only way......
>
>
> On Fri, Jan 4, 2013 at 6:29 PM, anish singh <anish198519851985 at gmail.com>wrote:
>
>> On Fri, Jan 4, 2013 at 3:41 PM, Rajat Sharma <fs.rajat at gmail.com> wrote:
>> >> Is this correct for all architectures?
>> >
>> > I guess not, asmlinkage is undefined for arm, so I assume this
>> mechanism is
>> > not there for arm.
>> then how do they do it?
>> >
>> >
>> >
>> > On Fri, Jan 4, 2013 at 2:24 PM, 卜弋天 <buyit at live.cn> wrote:
>> >>
>> >>
>> >>
>> >> 在 2013-1-4，15:38，"Rajat Sharma" <fs.rajat at gmail.com> 写道：
>> >>
>> >> > So with asmlinkage we request compiler to put args on stack. What is
>> >> > advantage of this to start_kernel or in general to other functions ?
>> >>
>> >> See its about implementation ease and little of performance too.
>> Assuming
>> >> the default model of keeping arguments in registers is used. lets say
>> >> arguments are assumed to be in registers R1, R2, R3, R4, R5, R6 and
>> beyond
>> >> that in stack. Since system call number is a transparent argument
>> which is
>> >> chopped off when calling the actual kernel handler and if R1 had the
>> system
>> >> call number, then you have to shift all register values and stack
>> arguments
>> >> too.
>> >>
>> >>    Is this correct for all architectures?
>> >>
>> >>    As I remembered, ARM uses SWI instruction to implement the system
>> call,
>> >> it will pass system call number by register R7, and use normal register
>> >> R0~R3 to pass parameters.
>> >>
>> >>
>> >>
>> >> Now consider that all arguments are pushed on stack (as enforced by
>> >> asmlinkage), you have all function argument in the beginning of the
>> stack
>> >> and the system call number on top of the stack. you just need to pop
>> out
>> >> stack top to remove system call number from function argument.
>> >>
>> >> You might argue that why not always keep system call number on stack
>> top
>> >> and use registers for function arguments? But thats part of the
>> compiler ABI
>> >> and if you had fewer arguments lets say 2 only and used up R1 and R2
>> only,
>> >> you may not jump to stack top directly for storing system call as its
>> turn
>> >> for R3 as argument.
>> >>
>> >> So, isn't it simpler implementation with everything on stack?
>> >>
>> >> -Rajat
>> >>
>> >>
>> >> On Fri, Jan 4, 2013 at 12:13 PM, Rahul Bedarkar <rpal143 at gmail.com>
>> wrote:
>> >>>
>> >>> Thanks. So with asmlinkage we request compiler to put args on stack.
>> What
>> >>> is advantage of this to start_kernel or in general to other functions
>> ?
>> >>>
>> >>> Regards,
>> >>> Rahul
>> >>>
>> >>>
>> >>> On Thu, Jan 3, 2013 at 9:34 PM, Mulyadi Santosa
>> >>> <mulyadi.santosa at gmail.com> wrote:
>> >>>>
>> >>>> On Thu, Jan 3, 2013 at 7:40 PM, Rahul Bedarkar <rpal143 at gmail.com>
>> >>>> wrote:
>> >>>> > Hi,
>> >>>> >
>> >>>> > I was searching for asmlinkage and found that it is already
>> explained
>> >>>> > at
>> >>>> > http://kernelnewbies.org/FAQ/asmlinkage
>> >>>> >
>> >>>> > But I didn't get this. Can someone tell me about it in brief ?
>> >>>>
>> >>>> the point is, parameters which is usually passed via stack, is passed
>> >>>> using different way.
>> >>>>
>> >>>> A good example is system call.... they are passed using registers
>> IIRC
>> >>>>
>> >>>>
>> >>>> --
>> >>>> regards,
>> >>>>
>> >>>> Mulyadi Santosa
>> >>>> Freelance Linux trainer and consultant
>> >>>>
>> >>>> blog: the-hydra.blogspot.com
>> >>>> training: mulyaditraining.blogspot.com
>> >>>
>> >>>
>> >>>
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>
>
>
> --
> Regards,
> Peter Teoh
>
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