RE: inline asm question(s)

["manish regmi" <manish_regmi@xxxxxxxxxxx>]

> Hi there people,
> I'm trying to figure out for several days now how the following functions:
>
> <from include/asm-i386/thread_info.h>
>  86 /* how to get the thread information struct from C */
>  87 static inline struct thread_info *current_thread_info(void)
>  88 {
>  89         struct thread_info *ti;
>  90         __asm__("andl %%esp,%0; ":"=r" (ti) : "" (~(THREAD_SIZE - 
> 1)));
>  91         return ti;
>  92 }
>
> I've read several tutorials on inline asm but what is peculiar here is, 
> that
> the input operand's constraint is "" (nothing) whereas the 
> (~(THREAD_SIZE-1))
> = (~(8192-1)) = (~0001 1111 1111 1111) = 1110 0000 0000 0000, IIRC, gets
> passed as a C arg.
> What exactly happens here?

You missed 0 -> "0".
It is the 2.6 kernel's way of getting the thread_info structure of the 
currently running task.

It works exactly current worked in 2.4s.
/* in 2.4 current.h*/
static inline struct task_struct * get_current(void)
{
	struct task_struct *current;
	__asm__("andl %%esp,%0; ":"=r" (current) : "0" (~8191UL));
	return current;
}

The principle is simple. Each task has THREAD_SIZE sized kernel stack. At 
the low end of that stack, there is a pointer to the thread_info.
So, to get the thread_info of the current task, round to the base of current 
kernel stack(by clearing the lower bits.) whiere lies the pointer to the 
current task's task_info.

Entry.S does the same thing with
#define GET_THREAD_INFO(reg) \
	movl $-THREAD_SIZE, reg; \
	andl %esp, reg


regards manish

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