What is a kernel stack used for?

Question

The following is a description I read of a context switch between process A and process B. I don't understand what a kernel stack is used for. There is suppose to be a per process kernel stack. And the description I am reading speaks of saving registers of A onto the kernel stack of A and also saving registers of A to the process structure of A. What exactly is the point of saving the registers to both the kernel stack and the process structure and why the need for both?

A context switch is conceptually simple: all the OS has to do is save a few register values for the currently-executing process (onto its kernel stack, for example) and restore a few for the soon-to-be-executing process (from its kernel stack). By doing so, the OS thus ensures that when the return-from-trap instruction is finally executed, instead of returning to the process that was running, the system resumes execution of another process...

Process A is running and then is interrupted by the timer interrupt. The hardware saves its registers (onto its kernel stack) and enters the kernel (switching to kernel mode). In the timer interrupt handler, the OS decides to switch from running Process A to Process B. At that point, it calls the switch() routine, which carefully saves current register values (into the process structure of A), restores the registers of Process B (from its process structure entry), and then switches contexts, specifically by changing the stack pointer to use B’s kernel stack (and not A’s). Finally, the OS returns-from-trap, which restores B’s registers and starts running it.

Answer 1

I have a disagreement with the second paragraph.

Process A is running and then is interrupted by the timer interrupt. The hardware saves its registers (onto its kernel stack) and enters the kernel (switching to kernel mode).

I am not aware of a system that saves all the registers on the kernel stack on an interrupt. Program Counter, Processor Status, and Stack Pointer (assuming the hardware does not have a separate Kernel Mode Stack Pointer). Normally, processors save the minimum necessary on the kernel stack after an interrupt. The interrupt handler will then save any additional registers it wants to use and restores them before exit. The processor's RETURN FROM INTERRUPT or EXCEPTION instruction then restores the registers automatically stored by the interrupt.

That description assumes no change in the process.

If the interrupt handle decides to change the process, it saves the current register state (the "process context" --most processors have a single instruction for this. In Intel land you might have to use multiple instructions) then executes another instruction to load the process context of the new process.

To answer your heading question "What is a kernel stack used for?", it is used whenever the processor is in Kernel mode. If the kernel did not have a stack protected from user access, the integrity of the system could be compromised. The kernel stack tends to be very small.

To answer you second question, "What exactly is the point of saving the registers to both the kernel stack and the process structure and why the need for both?"

They serve two different purpose. The saved registers on the kernel stack are used to get out of kernel mode. The context process block saves the entire register set in order to change processes.

I think your misunderstanding comes from the wording of your source that suggests all registers are stored on the stack when entering kernel mode, rather than just the minimum number of registers needed to make the kernel mode switch. The system will usually only save what it needs to get back to user mode (and may use that same information to return back to the original process in another context switch, depending upon the system). The change in process context saves all the registers.

Edits to answer additional questions:

If the interrupt handler needs to use register not saved by the CPU automatically by the interrupt, it pushes them on the kernel stack on entry and pops them off on exit. The interrupt handler has to explicitly save and restore any [general] registers it uses. The Process Context Block does not get touched for this.

The Process Context Block only gets altered as part an actual context switch.

Example:

Lets assume we have a processor with a program counter, stack pointer, processor status and 16 general registers (I know no such system really exists) and that the same SP is used for all modes.

1. Interrupt occurs.

The hardware pushes the PC, SP, and PS on to the stack, loads the SP with the address of the kernel mode stack and the PC from the interrupt handler (from the processor's dispatch table).

2. Interrupt handler gets called.

The writer of the handler decides he is going to us R0-R3. So the first lines of the handler have:

Push R0  ; on to the kernel mode stack
Push R1
Push R2
Push R3

3. The interrupt handler does whatever it wants to do.

4. Cleanup

The writer of the interrupt handler needs to do:

Pop R3
Pop R2
Pop R1
Pop R0 
REI      ; Whatever the system's return from interrupt or exception instruction is.

5. Hardware Takes over

Restores the PS, PC, and SP from the kernel mode stack, then resumes executing where it was before the interrupt.

I've made up my own processor for simplification. Some processors have lengthy instructions that are interruptable (e.g. block character moves). Such instructions often use registers to maintain their context. On such a system, the processor would have to automatically save any registers is uses to maintain context within the instruction.

An interrupt handler does not muck with the process context block unless it is changing processes.