Re: [LIP] A few queries

On Mon, Jun 07, 2004 at 11:38:01AM +0530, suresh kumar wrote:
> 
> Hi,
> 1)Can kernel directly access the user space address. If it can,
> why do we need the macros copy_from_user, copy_to_user etc...
> If it can'y why is it so? I have seen in the system call

It can. But it needs to be careful, because the user can pass a bad
pointer. In such cases, the kernel needs to check the access and return
-EFAULT. A bad pointer dereference from the kernel causes an oops.

> implementation on x86, eax has the system call index,
> ebx,ecx,edx,esi,edi are the registers that has
> parameters to the system call. If the no of parametres
> are more than five, then they are stored in a structure 
> and address of the structure is passed in one of the registers.
> Which means to my understanding, we are passing the address in
> the user space to the kernel.

Yes, that's the i386 syscall calling convention.

> 2) I read in lot many places, that two tasklets of the same type
> can not run simultaneously. But I haven't seen ne type of the
> tasklet defined. Does the type mean one of the 4 softirqs. Is it 
> like if a keyboard driver raises a tasklet of type TASKLET_SOFTIRQ, 
> a sound card diver also raises a tasklet of type TASKLET_SOFTIRQ, 
> only one of them will be running and not both a the same time on
> SMP. I just want to know is that true. 
> Or is it like, two tasklet from the same driver can't be run
> simultaneously on diff processors on SMP.
> Correct my understanding.

The I-ll-do-it-later reference I sent to the list earlier should answer
these questions ?

TASKLET_SOFTIRQ is not a type of tasklet. It's a type of softirq. If
you search for tasklet_init() in the kernel sources, you'll find about
60 hits as of 2.6.6. Each one of them is a type of tasklet.

	-Arun


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