I have already started all the processors. As I understand I need to load different TSS to every processor. But what exactly the TSS is doing? Should I need to use one for every process/thread?

Hi,



When a CPU switches from CPL=3 to CPL=0 (from user-space to kernel space) the CPU normally loads the "hopefully guaranteed safe to use" kernel stack from the TSS to make sure that malicious user-space code can't do something like setting its stack to an address in kernel space and tricking the kernel into trashing itself. Note: The SYSCALL instruction is a special case where the CPU does not switch to a safe stack.

At a minimum, each CPU will need to use different RAM for its kernel stack (otherwise you get problems when 2 or more CPUs are running kernel code at the same time). Beyond that; it depends on other design decisions (if the kernel uses hardware multi-tasking or software multi-tasking, if the kernel uses "one kernel stack per task" or "one kernel stack per CPU", if the kernel uses paging to make some of kernel space contain different things for different CPUs).

The typical case is "software multi-tasking, one kernel stack per task, all of kernel space is the same for all CPUs"; and in this case you'd need one TSS per CPU, where you'd set the "SS0:ESP0" fields in each CPU's TSS during boot/kernel initialisation.


Cheers,

Brendan