What is asmlinkage ?

[Rajat Sharma]

> Is this correct for all architectures?

I guess not, asmlinkage is undefined for arm, so I assume this mechanism is
not there for arm.



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
>>>
>>
>>
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies at kernelnewbies.org
>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>
>>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20130104/70642d02/attachment.html