From c71b6c6c8a2297b421e17e7fd53dc4cf654fb7f1 Mon Sep 17 00:00:00 2001
From: Stefan Lankes <lankes@lfbs.rwth-aachen.de>
Date: Wed, 22 Jan 2014 12:07:29 +0100
Subject: [PATCH] Simplify code, add additional comments

---
 arch/x86/include/asm/tasks.h | 3 ---
 arch/x86/kernel/entry.asm    | 5 +++++
 arch/x86/kernel/gdt.c        | 6 ++++--
 kernel/main.c                | 4 +---
 4 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/tasks.h b/arch/x86/include/asm/tasks.h
index 2fae22a..e7e2ca8 100644
--- a/arch/x86/include/asm/tasks.h
+++ b/arch/x86/include/asm/tasks.h
@@ -61,9 +61,6 @@ void switch_context(size_t** stack);
  */
 int create_default_frame(task_t* task, entry_point_t ep, void* arg);
 
-/** @brief Copy kernel stack pointer to TSS */
-void set_kernel_stack(size_t stack);
-
 /** @brief Register a task's TSS at GDT
  *
  * @return
diff --git a/arch/x86/kernel/entry.asm b/arch/x86/kernel/entry.asm
index 08a4c26..19df613 100644
--- a/arch/x86/kernel/entry.asm
+++ b/arch/x86/kernel/entry.asm
@@ -263,6 +263,8 @@ ALIGN 4
 rollback:
     ret
 
+extern set_kernel_stack
+
 ALIGN 4
 common_stub:
     pusha
@@ -290,6 +292,9 @@ common_switch:
     or eax, 8
     mov cr0, eax
 
+    ; set esp0 in the task state segment
+    call set_kernel_stack
+
     ; call cleanup code
     call finish_task_switch
 
diff --git a/arch/x86/kernel/gdt.c b/arch/x86/kernel/gdt.c
index 3f23539..43190af 100644
--- a/arch/x86/kernel/gdt.c
+++ b/arch/x86/kernel/gdt.c
@@ -46,9 +46,11 @@ static gdt_entry_t		gdt[GDT_ENTRIES] = {[0 ... GDT_ENTRIES-1] = {0, 0, 0, 0, 0,
  */
 extern void gdt_flush(void);
 
-void set_kernel_stack(size_t stack)
+void set_kernel_stack(void)
 {
-	task_state_segment.esp0 = stack;
+	task_t* curr_task = current_task;
+
+	task_state_segment.esp0 = (size_t) curr_task->stack + KERNEL_STACK_SIZE-16;
 }
 
 /* Setup a descriptor in the Global Descriptor Table */
diff --git a/kernel/main.c b/kernel/main.c
index 429ae49..34b8642 100644
--- a/kernel/main.c
+++ b/kernel/main.c
@@ -65,9 +65,7 @@ static int wrapper(void* arg)
 
 	memset(ustack, 0xCD, KERNEL_STACK_SIZE);
 	*stack-- = (size_t) arg;
-	*stack = (size_t) leave_user_task; // add dummy return value
-
-	set_kernel_stack((size_t) &stack);
+	*stack = (size_t) leave_user_task; // put exit function as caller on the stack
 
 	return jump_to_user_code((uint32_t) userfoo, (uint32_t) stack);
 }