From 42e92e7e9ab971acd6a1a55e38c0f4e999c1293c Mon Sep 17 00:00:00 2001
From: Alex Auvolat <alex.auvolat@ens.fr>
Date: Sun, 7 Dec 2014 20:26:57 +0100
Subject: Optimize context switching code on IRQ0.

---
 kernel/l0/context_switch.s | 32 ++++++++++++++++++++++----------
 kernel/l0/task.c           | 40 ++++++++++++++++++++++++++++++----------
 2 files changed, 52 insertions(+), 20 deletions(-)

diff --git a/kernel/l0/context_switch.s b/kernel/l0/context_switch.s
index 7351997..6738a03 100644
--- a/kernel/l0/context_switch.s
+++ b/kernel/l0/context_switch.s
@@ -11,10 +11,7 @@ save_context_and_enter_scheduler:
 	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+44]		; get address of saved_context structure
 	mov [eax], esp			; save esp
 	mov dword [eax+4], resume_saved_context		; save eip
 
@@ -22,12 +19,6 @@ save_context_and_enter_scheduler:
 	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
 
@@ -35,6 +26,27 @@ resume_saved_context:
 	popf
 	ret
 
+[GLOBAL irq0_save_context_and_enter_scheduler]
+; meant to be called on IRQ0
+; general registers already saved by IRQ handler stub
+; flags already saved by interruption and interruptions disabled
+; only saves CR3
+irq0_save_context_and_enter_scheduler:
+	mov eax, cr3
+	push eax
+
+	mov eax, [esp+8]		; get address of saved_context structure
+	mov [eax], esp			; save esp
+	mov dword [eax+4], resume_saved_irq0_context		; save eip
+
+	mov esp, kernel_stack_top
+	jmp run_scheduler
+
+resume_saved_irq0_context:
+	pop eax
+	mov cr3, eax
+	ret
+
 
 [GLOBAL resume_context]
 resume_context:
diff --git a/kernel/l0/task.c b/kernel/l0/task.c
index f8750e6..e31a85c 100644
--- a/kernel/l0/task.c
+++ b/kernel/l0/task.c
@@ -7,6 +7,7 @@
 #include <paging.h>
 
 void save_context_and_enter_scheduler(saved_context_t *ctx);
+void irq0_save_context_and_enter_scheduler(saved_context_t *ctx);
 void resume_context(saved_context_t *ctx);
 
 task_t *current_task = 0;
@@ -27,6 +28,21 @@ void set_pit_frequency(uint32_t freq) {
    outb(0x40, h);
 }
 
+// ============================= //
+// HELPER : IF FLAG MANIPULATION //
+// ============================= //
+
+static inline bool disable_interrupts() {
+	uint32_t eflags;
+	asm volatile("pushf; pop %0" : "=r"(eflags));
+	asm volatile("cli");
+	return (eflags & EFLAGS_IF) != 0;
+}
+
+static inline void resume_interrupts(bool st) {
+	if (st) asm volatile("sti");
+}
+
 // ================== //
 // THE TASK SCHEDULER //
 // ================== //
@@ -63,10 +79,13 @@ task_t* dequeue_task() {
 // ================ //
 
 void run_scheduler() {
+	// At this point, interrupts are disabled
 	// 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);
@@ -134,9 +153,13 @@ task_t *new_task(entry_t entry) {
 	return t;
 }
 
+void irq0_handler(registers_t *regs) {
+	if (current_task != 0)
+		irq0_save_context_and_enter_scheduler(&current_task->ctx);
+}
 void tasking_setup(entry_t cont, void* arg) {
 	set_pit_frequency(TASK_SWITCH_FREQUENCY);
-	idt_set_irq_handler(IRQ0, yield);
+	idt_set_irq_handler(IRQ0, irq0_handler);
 
 	task_t *t = new_task(cont);
 	ASSERT(t != 0);
@@ -158,9 +181,7 @@ void yield() {
 }
 
 void* wait_for_result() {
-	uint32_t eflags;
-	asm volatile("pushf; pop %0" : "=r"(eflags));
-	asm volatile("cli");
+	bool st = disable_interrupts();
 
 	if (!current_task->has_result) {
 		current_task->state = T_STATE_WAITING;
@@ -169,15 +190,14 @@ void* wait_for_result() {
 	ASSERT(current_task->has_result);
 	current_task->has_result = false;
 
-	if (eflags & EFLAGS_IF) asm volatile("sti");
+	void *result = current_task->result;
 
-	return current_task->result;
+	resume_interrupts(st);
+	return 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");
+	bool st = disable_interrupts();
 
 	task->has_result = true;
 	task->result = data;
@@ -188,7 +208,7 @@ void resume_with_result(task_t *task, void* data, bool run_at_once) {
 	}
 	if (run_at_once) yield();
 
-	if (eflags & EFLAGS_IF) asm volatile("sti");
+	resume_interrupts(st);
 }
 
 /* vim: set ts=4 sw=4 tw=0 noet :*/
-- 
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