Hi, The SS is the same for all the processes. SS is initialized once in INIT_TSS but the process esp is maintained separately Neil has given the soln.Just have a look : There is no such thing as "the common kernel stack". Each process (represented by a task_struct in the kernel) has its own private data space to be used as a kernel stack when that process traps into the kernel. You can see where this per task_struct stack space is reserved in the definition of task_union. init_[task|thread]_union just defines the first task union in the system. Because of the way unions are laid out in memory, The kernel knows that when a process traps into kernel space, it just needs to round the current task pointer to the nearest 8k (prehaps 4k in 2.6) boundary, and thats the start of that processes kernel stack. Thats how the SAVE_ALL command avoids trampling registers. HTH Neil Add anything to this if u feel its necessary Regards Suthambhara On Wed, 13 Oct 2004 10:05:15 +0530, Dhiman, Gaurav <gaurav.dhiman@xxxxxx> wrote: > > You discussed that kernel do not keep track of SS for process specific > kernel stack as it always starts from fixed offset of task_struct, but > does that mean, linux kernel do not make use of SS element in the TSS of > process. I think the kernel can not by-pass the rules defined by > processor. Processor expects the SS and SP elements to be right at the > time of stack switching, so we need to initialize them to the right > values while forking a process. > > I think kernel definitely keep tracks of SS and SP for all CPU levels > including kernel mode also (ring 0), so that when we are switching form > user space to kernel space the processor can switch the stacks > automatically. At stack switching time CPU expects the SS and SP > elements of TSS to be right, it's just going to copy those values in SS > and SP registers of CPU, so that now we can push and pop the things on > the kernel stack. > > Yes it definitely can happen (and I think this is the way to do it) that > SS in process's TSS is initialized to the memory address just next to > task_struct (as the kernel stack can grow downwards till this limit) > while the forking of a process and also sets the SP to the fixed offset > from task_struct from where the kernel stack starts growing downwards > (as you mentioned kernel stack starts from there). Now whenever this > process will be scheduled by the scheduler or a process enters the > kernel mode, CPU's SS and SP registers are re-set to the SS and SP > elements in TSS of that process. > > At the time of scheduling, scheduler also change the TSS descriptor for > that CPU on which the process is going to be scheduled. It changes this > TSS descriptor in GDT to point to the TSS of the selected process. > > Correct me if I am wrong. > > Cheers !! > Gaurav > > > > > -----Original Message----- > From: Jan Hudec [mailto:bulb@xxxxxxxxxxxxxxxxxx] On Behalf Of Jan Hudec > Sent: Tuesday, October 12, 2004 6:42 PM > To: aq > Cc: suthambhara nagaraj; Dhiman, Gaurav; main kernel; kernel > Subject: Re: Kernel stack > > On Tue, Oct 12, 2004 at 21:30:54 +0900, aq wrote: > > > > >From what you all discuss, I can say: kernel memory is devided > into 2 > > > > part, and the upper part are shared between processes. The below > part > > > > (the kernel stack, or 8K traditionally) is specifict for each > process. > > > > > > > > Is that right? > > > > > > No, it's not. There is just one kernel memory. In it each process > has > > > it's own task_struct + kernel stack (by default 8K). There is no > special > > > address mapping for these, nor are they allocated from a special > area. > > > > > > When a context of some process is entered, esp is pointed to the top > of > > > it's stack. That's exactly all it takes to exchange stacks. > > > > OK, lets say there are 20 processes running in the system. Then the > > kernel must allocate 20 * 8K = 160K just for the stacks of these > > processes. All of these 160K always occupy the kernel (kernel memory > > is never swapped out). When a process actives, ESP would switch to > > point to the corresponding stack (of that process). > > This is correct. > > > The remainding memory of kernel therefore is equally accessible to all > > the processes. > > This is not. There is nothing like "remaining memory". **ALL* kernel > memory is equally accessible to all the processes. > > There is noting special about the stacks and task-structs. They are > normal 8K structures somewhere in kernel memory. > > > Is that correct ? > > ------------------------------------------------------------------------ > ------- > Jan 'Bulb' Hudec > <bulb@xxxxxx> > -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
