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src/exp6/include/sched.h

@@ -73,6 +73,9 @@ extern void switch_to(struct task_struct* next);
 extern void cpu_switch_to(struct task_struct* prev, struct task_struct* next);
 extern void exit_process(void);
 
+// the initial values for task_struct that belongs to the init task. see sched.c 
+// NB: init task is in kernel, only has kernel mapping (ttbr1) 
+// 		no user mapping (ttbr0, mm->pgd=0)
 #define INIT_TASK \
 /*cpu_context*/ { { 0,0,0,0,0,0,0,0,0,0,0,0,0}, \
 /* state etc */	 0,0,15, 0, PF_KTHREAD, \

src/exp6/src/fork.c

@@ -30,7 +30,8 @@ int copy_process(unsigned long clone_flags, unsigned long fn, unsigned long arg)
 	p->state = TASK_RUNNING;
 	p->counter = p->priority;
 	p->preempt_count = 1; //disable preemtion until schedule_tail
-
+	// @page is 0-filled, many fields (e.g. mm.pgd) are implicitly init'd
+
 	p->cpu_context.pc = (unsigned long)ret_from_fork;
 	p->cpu_context.sp = (unsigned long)childregs;
 	int pid = nr_tasks++;
@@ -41,7 +42,11 @@ int copy_process(unsigned long clone_flags, unsigned long fn, unsigned long arg)
 }
 
 
-/* @start: a pointer to the beginning of the user code (to be copied to the new task), 
+/* 
+	Populate pt_regs for returning to user space (via kernel_exit) for the 1st time. 
+   	Note that the actual switch will not happen until kernel_exit. 
+
+	@start: a pointer to the beginning of the user code (to be copied to the new task), 
    @size: size of the area 
    @pc: offset of the startup function inside the area
 */   
@@ -51,7 +56,10 @@ int move_to_user_mode(unsigned long start, unsigned long size, unsigned long pc)
 	struct pt_regs *regs = task_pt_regs(current);
 	regs->pstate = PSR_MODE_EL0t;
 	regs->pc = pc;
+	/* assumption: our toy user program will not exceed 1 page. the 2nd page will serve as the stack */
 	regs->sp = 2 *  PAGE_SIZE;  
+	/* only allocate 1 code page here b/c the stack page is to be mapped on demand. 
+	   this will trigger allocating the task's pgtable tree (mm.pgd) */
 	unsigned long code_page = allocate_user_page(current, 0);
 	if (code_page == 0)	{
 		return -1;
@@ -61,6 +69,9 @@ int move_to_user_mode(unsigned long start, unsigned long size, unsigned long pc)
 	return 0;
 }
 
+/* get a task's saved registers, which are at the top of the task's kernel page. 
+   these regs are saved/restored by kernel_entry()/kernel_exit(). 
+*/
 struct pt_regs * task_pt_regs(struct task_struct *tsk)
 {
 	unsigned long p = (unsigned long)tsk + THREAD_SIZE - sizeof(struct pt_regs);