sched: add kernel-mode context switching

arch/x86_64/thread_switch.S

@@ -0,0 +1,54 @@
+	.code64
+
+	.extern THREAD_sp
+//TASK_threadsp:
+	//.long 32
+	
+	.global switch_to
+	.type switch_to, @function
+
+	// %rdi = (struct thread *) current thread.
+	// %rsi = (struct thread *) next thread.
+switch_to:
+	pushfq
+
+	push %rax
+	push %rcx
+	push %rdx
+	push %rbx
+	pushq $0
+	push %rbp
+	push %rsi
+	push %rdi
+	push %r8
+	push %r9
+	push %r10
+	push %r11
+	push %r12
+	push %r13
+	push %r14
+	push %r15
+	
+	movq %rsp, THREAD_sp(%rdi)
+	movq THREAD_sp(%rsi), %rsp
+
+	pop %r15
+	pop %r14
+	pop %r13
+	pop %r12
+	pop %r11
+	pop %r10
+	pop %r9
+	pop %r8
+	pop %rdi
+	pop %rsi
+	pop %rbp
+	add $8, %rsp
+	pop %rbx
+	pop %rdx
+	pop %rcx
+	pop %rax
+
+	popfq
+
+	ret