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@xxxxxx> 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@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/
