sched: implement user-configurable fs and gs segment base addresses

arch/x86_64/thread_switch.S

@@ -13,6 +13,22 @@ ml_thread_switch:
 	push %rax
 	push %rcx
 	push %rdx
+
+	// set fs-base
+	mov $0xC0000100, %rcx
+	movq THREAD_fsbase(%rsi), %rax
+	movq THREAD_fsbase(%rsi), %rdx
+	shr $32, %rdx
+	wrmsr
+
+	// set (kernel) gs-base (it will be swapped back into user-gs-base at
+	// the end of this function)
+	mov $0xC0000102, %rcx
+	movq THREAD_gsbase(%rsi), %rax
+	movq THREAD_gsbase(%rsi), %rdx
+	shr $32, %rdx
+	wrmsr
+
 	push %rbx
 	pushq $0
 	push %rbp