Implement multitasking.

5 files changed, 268 insertions, 21 deletions

diff --git a/kernel/l0/context_switch.s b/kernel/l0/context_switch.s
new file mode 100644
index 0000000..7351997
--- /dev/null
+++ b/kernel/l0/context_switch.s
@@ -0,0 +1,46 @@
+[EXTERN kernel_stack_top]
+[EXTERN run_scheduler]
+
+[GLOBAL save_context_and_enter_scheduler]
+; void save_context_and_enter_scheduler(struct saved_context *ctx);
+save_context_and_enter_scheduler:
+	pushf
+	cli
+	pusha					; Pushes edi,esi,ebp,esp,ebx,edx,ecx,eax
+
+	mov eax, cr3
+	push eax
+
+	mov ax, ds              ; Lower 16-bits of eax = ds.
+	push eax                ; save the data segment descriptor
+	
+	mov eax, [esp+48]		; get address of saved_context structure
+	mov [eax], esp			; save esp
+	mov dword [eax+4], resume_saved_context		; save eip
+
+	mov esp, kernel_stack_top
+	jmp run_scheduler
+
+resume_saved_context:
+	pop eax
+	mov ds, ax
+	mov es, ax
+	mov fs, ax
+	mov gs, ax
+
+	pop eax
+	mov cr3, eax
+
+	popa
+	popf
+	ret
+
+
+[GLOBAL resume_context]
+resume_context:
+	mov eax, [esp+4]		; get address of saved context
+	mov esp, [eax]			; resume esp
+	mov ecx, [eax+4]		; jump to specified eip
+	jmp ecx
+
+; vim: set ts=4 sw=4 tw=0 noet :
diff --git a/kernel/l0/kmain.c b/kernel/l0/kmain.c
index e6831b2..0a6bed9 100644
--- a/kernel/l0/kmain.c
+++ b/kernel/l0/kmain.c
@@ -10,6 +10,8 @@
 #include <region.h>
 #include <kmalloc.h>
 
+#include <task.h>
+
 #include <slab_alloc.h>
 
 extern char k_end_addr;	// defined in linker script : 0xC0000000 plus kernel stuff
@@ -19,11 +21,6 @@ void breakpoint_handler(registers_t *regs) {
 	BOCHS_BREAKPOINT;
 }
 
-void yield() {
-	// multitasking not implemented yet
-	dbg_printf("Warning : probable deadlock?\n");
-}
-
 void region_test1() {
 	void* p = region_alloc(0x1000, REGION_T_HW, 0);
 	dbg_printf("Allocated one-page region: 0x%p\n", p);
@@ -102,6 +99,28 @@ void kmalloc_test(void* kernel_data_end) {
 	dbg_printf("Kmalloc test OK.\n");
 	dbg_print_region_stats();
 }
+
+void test_task(void* a) {
+	int i = 0;
+	while(1) {
+		dbg_printf("b");
+		for (int x = 0; x < 100000; x++) asm volatile("xor %%ebx, %%ebx":::"%ebx");
+		if (++i == 8) {
+			yield();
+			i = 0;
+		}
+	}
+}
+void kernel_init_stage2(void* data) {
+	task_t *tb = new_task(test_task);
+	resume_with_result(tb, 0, false);
+
+	while(1) {
+		dbg_printf("a");
+		for (int x = 0; x < 100000; x++) asm volatile("xor %%ebx, %%ebx":::"%ebx");
+	}
+	PANIC("Reached kmain end! Falling off the edge.");
+}
  
 void kmain(struct multiboot_info_t *mbd, int32_t mb_magic) {
 	dbglog_setup();
@@ -141,8 +160,11 @@ void kmain(struct multiboot_info_t *mbd, int32_t mb_magic) {
 	kmalloc_setup();
 	kmalloc_test(kernel_data_end);
 
-
-	PANIC("Reached kmain end! Falling off the edge.");
+	// enter multi-tasking mode
+	// interrupts are enabled at this moment, so all
+	// code run from now on should be preemtible (ie thread-safe)
+	tasking_setup(kernel_init_stage2, 0);
+	PANIC("Should never come here.");
 }
 
 /* vim: set ts=4 sw=4 tw=0 noet :*/
diff --git a/kernel/l0/loader.s b/kernel/l0/loader.s
index 5d0a2b8..447d82d 100644
--- a/kernel/l0/loader.s
+++ b/kernel/l0/loader.s
@@ -2,6 +2,7 @@
 [GLOBAL loader]           ; making entry point visible to linker
 [GLOBAL kernel_pd]   	  ; make kernel page directory visible
 [GLOBAL kernel_stack_protector] 		; used to detect kernel stack overflow
+[GLOBAL kernel_stack_top] 				; stack re-used by scheduler
 
 ; higher-half kernel setup
 K_HIGHHALF_ADDR     equ 0xC0000000
@@ -60,7 +61,7 @@ higherhalf:		; now we're running in higher half
 	mov dword [kernel_pd], 0
 	invlpg [0]
 
-	mov esp, stack_top          ; set up the stack
+	mov esp, kernel_stack_top   ; set up the stack
 
 	push eax                    ; pass Multiboot magic number
 	add ebx, K_HIGHHALF_ADDR    ; update the MB info structure so that it is in higher half
@@ -78,8 +79,8 @@ hang:
 align 0x1000
 kernel_stack_protector:
 	resb 0x1000			; as soon as we have efficient paging, we WON'T map this page
-stack_bottom:
+kernel_stack_bottom:
 	resb LOADER_STACK_SIZE
-stack_top:
+kernel_stack_top:
 
 ; vim: set ts=4 sw=4 tw=0 noet :
diff --git a/kernel/l0/region.c b/kernel/l0/region.c
index 3691747..a384a4d 100644
--- a/kernel/l0/region.c
+++ b/kernel/l0/region.c
@@ -333,17 +333,6 @@ region_info_t *find_region(void* addr) {
 // HELPER FUNCTIONS : SIMPLE PF HANDLERS ; FREEING FUNCTIONS //
 // ========================================================= //
 
-void stack_pf_handler(pagedir_t *pd, struct region_info *r, void* addr) {
-	if (addr < r->addr + PAGE_SIZE) {
-		dbg_printf("Stack overflow at 0x%p.", addr);
-		if (r->type & REGION_T_KPROC_STACK) dbg_printf("    (in kernel process stack)\n");
-		if (r->type & REGION_T_PROC_KSTACK) dbg_printf("    (in process kernel stack)\n");
-		dbg_print_region_stats();
-		PANIC("Stack overflow.");
-	}
-	default_allocator_pf_handler(pd, r, addr);
-}
-
 void default_allocator_pf_handler(pagedir_t *pd, struct region_info *r, void* addr) {
 	ASSERT(pd_get_frame(addr) == 0);	// if error is of another type (RO, protected), we don't do anyting
 
diff --git a/kernel/l0/task.c b/kernel/l0/task.c
new file mode 100644
index 0000000..e02dcf6
--- /dev/null
+++ b/kernel/l0/task.c
@@ -0,0 +1,189 @@
+#include <task.h>
+#include <kmalloc.h>
+#include <dbglog.h>
+#include <idt.h>
+
+#include <frame.h>
+#include <paging.h>
+
+void save_context_and_enter_scheduler(saved_context_t *ctx);
+void resume_context(saved_context_t *ctx);
+
+task_t *current_task = 0;
+
+// ====================== //
+// THE PROGRAMMABLE TIMER //
+// ====================== //
+
+void set_pit_frequency(uint32_t freq) {
+   uint32_t divisor = 1193180 / freq;
+   ASSERT(divisor < 65536);	// must fit on 16 bits
+
+   uint8_t l = (divisor & 0xFF);
+   uint8_t h = ((divisor >> 8) & 0xFF);
+
+   outb(0x43, 0x36);
+   outb(0x40, l);
+   outb(0x40, h);
+}
+
+// ================== //
+// THE TASK SCHEDULER //
+// ================== //
+
+static task_t *queue_first_task = 0, *queue_last_task = 0;
+
+void enqueue_task(task_t *t, bool just_ran) {
+	ASSERT(t->state == T_STATE_RUNNING);
+	if (queue_first_task == 0) {
+		queue_first_task = queue_last_task = t;
+		t->next_in_queue = 0;
+	} else if (just_ran) {
+		t->next_in_queue = 0;
+		queue_last_task->next_in_queue = t;
+		queue_last_task = t;
+	} else {
+		t->next_in_queue = queue_first_task;
+		queue_first_task = t;
+	}
+}
+
+task_t* dequeue_task() {
+	task_t *t = queue_first_task;
+	if (t == 0) return 0;
+
+	queue_first_task = t->next_in_queue;
+	if (queue_first_task == 0) queue_last_task = 0;
+
+	return t;
+}
+
+// ================ //
+// THE TASKING CODE //
+// ================ //
+
+void run_scheduler() {
+	// This function is expected NEVER TO RETURN
+	if (current_task != 0 && current_task->state == T_STATE_RUNNING) {
+		enqueue_task(current_task, true);
+	}
+	current_task = dequeue_task();
+	if (current_task != 0) {
+		resume_context(&current_task->ctx);
+	} else {
+		// Wait for an IRQ
+		asm volatile("sti; hlt");
+		// At this point an IRQ has happenned
+		// and has been processed. Loop around. 
+		run_scheduler();
+		ASSERT(false);
+	}
+}
+
+static void run_task(void (*entry)(void*)) {
+	ASSERT(current_task->state == T_STATE_RUNNING);
+	ASSERT(current_task->has_result);
+
+	current_task->has_result = false;
+
+	asm volatile("sti");
+	entry(current_task->result);
+
+	current_task->state = T_STATE_FINISHED;
+	// TODO : add job for deleting the task, or whatever
+	yield();	// expected never to return!
+	ASSERT(false);
+}
+task_t *new_task(entry_t entry) {
+	task_t *t = (task_t*)kmalloc(sizeof(task_t));
+	if (t == 0) return 0;
+
+	void* stack = region_alloc(KPROC_STACK_SIZE, REGION_T_KPROC_STACK, 0);
+	if (stack == 0) {
+		kfree(t);
+		return 0;
+	}
+
+	for (void* i = stack + PAGE_SIZE; i < stack + KPROC_STACK_SIZE; i += PAGE_SIZE) {
+		uint32_t f = frame_alloc(1);
+		if (f == 0) {
+			region_free_unmap_free(stack);
+			kfree(t);
+			return 0;
+		}
+		pd_map_page(i, f, true);
+	}
+
+	t->stack_region = find_region(stack);
+
+	t->ctx.esp = (uint32_t*)(t->stack_region->addr + t->stack_region->size);
+	*(--t->ctx.esp) = (uint32_t)entry;	// push first argument : entry point
+	*(--t->ctx.esp) = 0;		// push invalid return address (the run_task function never returns)
+
+	t->ctx.eip = (void(*)())run_task;
+	t->state = T_STATE_WAITING;
+	t->result = 0;
+	t->has_result = false;
+
+	t->more_data = 0;
+
+	return t;
+}
+
+void tasking_setup(entry_t cont, void* arg) {
+	set_pit_frequency(100);
+	idt_set_irq_handler(IRQ0, yield);
+
+	task_t *t = new_task(cont);
+	ASSERT(t != 0);
+
+	resume_with_result(t, arg, false);
+
+	run_scheduler();	// never returns
+	ASSERT(false);
+}
+
+void yield() {
+	if (current_task == 0) {
+		// might happen before tasking is initialized
+		dbg_printf("Warning: probable deadlock.");
+	} else {
+		save_context_and_enter_scheduler(&current_task->ctx);
+	}
+}
+
+void* wait_for_result() {
+	uint32_t eflags;
+	asm volatile("pushf; pop %0" : "=r"(eflags));
+	asm volatile("cli");
+
+	if (!current_task->has_result) {
+		current_task->state = T_STATE_WAITING;
+		save_context_and_enter_scheduler(&current_task->ctx);
+	}
+	ASSERT(current_task->has_result);
+	current_task->has_result = false;
+
+	if (eflags & EFLAGS_IF) asm volatile("sti");
+
+	return current_task->result;
+}
+
+void resume_with_result(task_t *task, void* data, bool run_at_once) {
+	uint32_t eflags;
+	asm volatile("pushf; pop %0" : "=r"(eflags));
+	asm volatile("cli");
+
+	task->has_result = true;
+	task->result = data;
+
+	if (task->state == T_STATE_WAITING) {
+		task->state = T_STATE_RUNNING;
+		enqueue_task(task, false);
+	}
+	if (run_at_once) yield();
+
+	if (eflags & EFLAGS_IF) asm volatile("sti");
+}
+
+/* vim: set ts=4 sw=4 tw=0 noet :*/