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

arch/x86_64/include/kernel/machine/thread.h

@@ -3,6 +3,10 @@
 
 #include <kernel/sched.h>
 
+struct ml_thread {
+	virt_addr_t tr_gsbase, tr_fsbase;
+};
+
 struct ml_cpu_context;
 
 /* switch from one thread to another. the stack of the `to` thread must have
@@ -28,4 +32,15 @@ extern kern_status_t ml_thread_prepare_user_context(
 	const uintptr_t *args,
 	size_t nr_args);
 
+extern kern_status_t ml_thread_config_get(
+	struct thread *thread,
+	kern_config_key_t key,
+	void *out,
+	size_t max);
+extern kern_status_t ml_thread_config_set(
+	struct thread *thread,
+	kern_config_key_t key,
+	const void *ptr,
+	size_t len);
+
 #endif