HIGH LEVEL, 5 STEPS TO BOOTSTRAP

0-hardware starts in kernel mode
1-configure hardware - mmu, install interrupt handler
2-configure current execution as 1st (0th) process (pid 0, init_task)
3-0th process creates next process (pid 1)
4-call schedule (save pc of current process, change pc to next process)
  schedule context switch to some other process from runqueue
  only way for a process to stop running is via schedule
5-pid 1 runs
n-when back in kernel code, kernel regains control, can schedule something else
  for example, timer interrupt goes off

IN MORE DETAIL, 5 STEPS IN LINUX BOOT TO INIT (ARM64)

0-kernel program starts
  arch/arm64/kernel/head.S
  efi -> primary_entry -> create_idmap, __primary_switch 

1-configure hardware, including virtual addressing
  __primary_switch -> __enable_mmu, create_kernel_mapping

2-configure current execution as 0th process (pid 0, init_task)
  set up data structure for 0th process
  __primary_switch -> __primary_switched -> init_cpu_task, start_kernel
  init_cpu_task does msr sp_el0 with init_task
  init_task statically defined init/init_task.c
  include/linux/sched.h struct task_struct, cpu-specific state in thread field
  arch/arm64/include/asm/processor.h struct thread_struct, has cpu_context
  arch/arm64/include/asm/current.h current is get_current from sp_el0
  also install handlers adr_l x8, vectors  and 	msr vbar_el1, x8
  vectors defined in arch/arm64/kernel/entry.S

  initialize other kernel data structures, including scheduler
  start_kernel -> sched_init in kernel/sched/core.c
  initialize scheduler on boot cpu (1 cpu), initializes cpu's rq, etc.
  sched_init -> init_idle in kernel/sched/core.c
  init_idle sets current task (ie init_task) to idle task for boot cpu
  sprintf(idle->comm, "%s/%d", INIT_TASK_COMM, cpu) updates
  comm to swapper/cpu# (ie swapper/0)

3-0th process "forks" 1st process to run kernel_init
  1st process put in runqueue
  init/main.c start_kernel -> arch_call_rest_init -> rest_init
  starts init user_mode_thread(kernel_init, NULL, CLONE_FS)
  kernel/fork.c user_mode_thread -> kernel_clone -> copy_thread
  arch/arm64/kernel/process.c copy_thread
  thread.cpu_context.pc = ret_from_fork, thread.cpu_context.x19 = kernel_init
  arm64/include/asm/processor.h struct cpu_context

4-0th process idle task, calls schedule
  init/main.c rest_init -> schedule_preempt_disabled, cpu_startup_entry
  kernel/sched/core.c schedule_preempt_disabled -> schedule
  cpu_startup_entry -> while(1) do_idle()
  do_idle loops if nothing to run, otherwise calls schedule_idle -> schedule

5-schedule context switches to some other process from runqueue (1st process)
  kernel/sched/core.c schedule -> __schedule -> context_switch -> switch_to
  include/asm-generic/switch_to.h switch_to -> __switch_to
  arch/arm64/kernel/process.c __switch_to -> cpu_switch_to
  arch/arm64/kernel/asm-offsets.c THREAD_CPU_CONTEXT thread.cpu_context
  arch/arm64/kernel/entry.S cpu_switch_to
  add x8, x1, x10 (next task_struct cpu_context)
  ldp instructions load values from cpu_context and update x8
  ldr lr, [x8] loads pc from cpu_context into linker register
  msr sp_el0, x1 (current to next task)
  ret returns to pc in linker register

  1st process will run and do ret_from_fork arch/arm64/kernel/entry.S
  ret_from_fork -> schedule_tail, x19 (kernel_init)
  kernel_init does kernel_init_freeable and run_init_process
  kernel_init_freeable -> prepare_namespace -> mount_root
  run_init_process -> kernel_execve using "/init" or "/sbin/init" (process 1)
  ret_from_fork -> ret_to_user -> kernel_exit arch/arm64/kernel/entry.S
  kernel_exit ends with eret

  init (systemd) regular program, reads config file, starts other processes

n-at some point, will return to kernel code
  when back in kernel code, kernel regains control, can schedule
  something else, etc.
  for example, system call causes function in kernel to run