CS 452/652 Winter 2023 - Lecture 5

Jan 24, 2023 prev next

Context Switch (cont'd)

After SVC

handler in exception vector executes
processor in EL1
ELR_EL1 holds return address
ESR_EL1 contains N from SVC
SPSR holds the processor state from before SVC
SPSel determines SP - set to 1 to avoid problems
- special access to pstate bits via MSR/MRS (Programmer's Guide Section 6.5.2)
C function call: compiler has saved x0-x18 (as needed)

Exception Handler

must save ELR - and later SPSR for IRQ
can access user SP using SPSel or directly as SP_EL0
has access to x0-x30
choices for storing user context:
- on user stack, or
- in task descriptor
then restore kernel context
symmetry with kernel entry and exit (syscall vs irq)?

Kernel Exit

store kernel context (similar to voluntary stack switch)
restore given user context (next task to be run)
return to user mode via eret

Additional Information

An earlier document by Bill Cowan is available here. This is certainly not the only way to write a context switch and I do not necessarily recommend (or not recommend) this particular approach, but I figure every bit of information can help.

Mixing C and Assembler

Coding

See various examples file in demo05. Take a look at the compiled .s source

main.c, adder.c - basic argument passing in r0, r1
adder10.c - beyond 8 arguments passed on stack
main.c, asm_adder.S - manual assembler routine working with C caller
main3.c, data_adder.c - memory-based data exchange between C and assembler
asm.c - embed assembler in C code; exchange data with local variables
- special register access: msr/mrs

Recommendations

use embedded assembler via asm only for short code sequences that do not access sp
use assembler routine linked as C routine for context-switch code

RPi Initialization

power cycle (10 seconds power-off): all memory reset, devices reset
regular exit (return) from kernel -> reboot

Kernel Start

EL1, working stack, IRQs disabled, caches?
set up (K1): SVC handler
set up (K2): caches
set up (later): timers, SPI/UART, IRQ handler, IRQs, track
create dedicated first user task (init task)
- add to ready queue
start kernel loop

Task Management

task creation: need to set up user stack to enable leave_kernel() functionality

handle end of user routine? use dedicate start routine

  void task_start(void (*task_function)()) {
    task_function();
    Exit();
  }

task termination: Exit() system call → kernel retires task
or add equivalent functionality to the fake initial user stack

Scheduling

preemption: each kernel entry triggers re-scheduling
- but: no time slicing
after kernel entry and corresponding processing
- task might be blocked → track
- task might be ready → back on ready queue
- but not both! single intrusive linkage element sufficient
ready queue - keep simple: array of plain FIFO queues
- no support for timeout/cancellation of blocking operations
- no need to remove internal element from queue
- only push to back of queue, pop from front