Hi Paul,
I'll give you two examples:
On PPC the current is simply defined as the register r2.
quoting from a vanilla 2.6.11 here (Linux/include/asm-ppc/current.h ) :
register struct task_struct *current asm ("r2");
This is possible due to the large number of registers in that architecture,
whereas in the not-so-lucky x86 architecture current is an inline function
which dereferences the task member out of the current thread_info struct.
( return current_thread_info()->task; )
current_thread_info is calculating the address of thread_info, btw, by
masking the 13 least significant bits of the stack pointer.
( assuming stack size is 8192 bytes size and the thread_info struct is
indeed living on its bottom).
"sacrificing" a register for the task is honorable, but not cheap :)
hope that helps,
Ohad.
On 8/29/05, Paul Duplys <kernel_newbie@xxxxxx> wrote:
> Hi all!
>
> As I already mentioned in my earlier emails, I am working on a linux port
> for a new architecture.
>
> I'm now trying to define the "current" macro. I read that most architectures
> keep a pointer to the thread_info instance in some processor register, so I
> thought it will be perhaps ok if I'll do it too.
>
> Well, that's actually not a problem, since I can get the content of any
> register using inline assembly instructions. But which register should I
> use?
>
> I mean, where in the kernel is a macro or a function which stores the
> pointer to thread_info in some register? In other words, how do I find out
> which register is actually used by the kernel for storing that pointer?
>
> I also want to thank you all for your answers to my earlier emails. You have
> really helped me a lot and I'm glad, that I joined this mailing list!
>
> Regards, Paul
>
>
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