Re: What is asmlinkage ?

[Rajat Sharma <fs.rajat@xxxxxxxxx>]

> 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@xxxxxxxxx> 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@xxxxxxxxx> wrote:

On Fri, Jan 4, 2013 at 3:41 PM, Rajat Sharma <fs.rajat@xxxxxxxxx> 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@xxxxxxx> wrote:
>>
>>
>>
>> 在 2013-1-4，15:38，"Rajat Sharma" <fs.rajat@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> wrote:
>>>>
>>>> On Thu, Jan 3, 2013 at 7:40 PM, Rahul Bedarkar <rpal143@xxxxxxxxx>
>>>> 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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>>
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--
Regards,
Peter Teoh

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