Re: Kernel stack

[suthambhara nagaraj <suthambhara@gmail.com>]

Hi,
The problem is each process does not have a TSS of its own.Only one
TSSper processor is present and the process dependant features (Like
esp) are stored
in another structure( struct thread_struct ).A kernel stack of size 8k
(By default)
is actully shared by processes running on a processor. There is a func named
load_tss (or something similiar) which loads the TSS from the
thread_struct structure during task switch .

A Process does not have an SS entry in its thread_struct but only an
esp (and esp0) entry. This made me believe that the stack base is the
same.
Correct me

Regards


On Tue, 12 Oct 2004 11:55:24 +0530, Dhiman, Gaurav <gaurav.dhiman@ca.com> wrote:
> 
> > I have not understood how the common kernel stack in the
> > init_thread_union(2.6 ,init_task_union in case of 2.4) works for all
> > the processes which run on the same processor
> 
> As far as I know, Kernel do not have any common stack for all the
> processes running over it. Whenever we enter the kernel mode thru system
> calls, we go thru system gate or descriptor (0x80 entry) in IDT. This
> entry contains the index of the descriptor in GDT (normally it points to
> Kernel CS Segment Descriptor in GDT) and the offset (pointer) to the
> code to be executed in kernel mode (which is system_call() function in
> Kernel).
> 
> Now the descriptor entry in GDT pointed out by the system gate entry in
> IDT, contains 2 bit field known as DPL (Desired Privelege Level). If
> this DPL is less than the CPL (Current Prevelege Level) of CPU then CPU
> switches to the process specific kernel stack segement by refferring the
> TSS of current running process. This stack switch is automatic by CPUand
> there is no assembly intruction required for it.
> 
> This stack switch is done at the time when we enter from user space to
> the kernel space, this is done because we can not trust and share the
> user process stack (stack used by user process in user mode). That is
> why every process has atleast two and can even have four stacks. In each
> process, stack for every CPU level (ring level) is defined. So whenever
> the process runs in user mode (ring 3), its user mode stack is used, but
> when it enters the kernel mode (ring 0) its stack is switched to the
> kernel stack of that process. All the stacks of a process for different
> levels of CPU are tracked thru TSS defined for that process.
> 
> To read more on IDT, GDT, TSS and System Calls invocation, refer to the
> Intels System Programmer's Guide. Her is the Link:
> ftp://download.intel.com/design/PentiumII/manuals/24319202.pdf
> 
> Correct me if I am wrong somewhere.
> 
> Cheers !!
> Gaurav
> 
> 
> 
> 
> -----Original Message-----
> From: kernelnewbies-bounce@nl.linux.org
> [mailto:kernelnewbies-bounce@nl.linux.org] On Behalf Of suthambhara
> nagaraj
> Sent: Tuesday, October 12, 2004 10:31 AM
> To: kernel
> Subject: Kernel stack
> 
> Hi all,
> 
> I have not understood how the common kernel stack in the
> init_thread_union(2.6 ,init_task_union in case of 2.4) works for all
> the processes which run on the same processor. The scheduling is round
> robin and yet the things on the stack (saved during SAVE_ALL) have to
> be maintained after a switch without them getting erased. I am
> familiar with only the i386 arch implementation.
> 
> Please help
> 
> regards,
> Suthambhara
> 
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