> 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@xxxxxxxxxxxx [mailto:kernelnewbies-bounce@xxxxxxxxxxxx] 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 -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/ -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
