sched: thread: implement cloning of userspace thread contexts

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

@@ -31,6 +31,12 @@ extern kern_status_t ml_thread_prepare_user_context(
 	virt_addr_t *kernel_sp,
 	const uintptr_t *args,
 	size_t nr_args);
+/* prepare the stack so that ml_thread_switch_user can jump to usermode
+ * with the specified register context */
+extern kern_status_t ml_thread_clone_user_context(
+	const struct ml_cpu_context *ctx,
+	uintptr_t return_value,
+	virt_addr_t *kernel_sp);
 
 extern kern_status_t ml_thread_config_get(
 	struct thread *thread,

arch/x86_64/thread.c

@@ -80,6 +80,21 @@ extern kern_status_t ml_thread_prepare_user_context(
 	return KERN_OK;
 }
 
+kern_status_t ml_thread_clone_user_context(
+	const struct ml_cpu_context *src_ctx,
+	uintptr_t return_value,
+	virt_addr_t *kernel_sp)
+{
+	(*kernel_sp) -= sizeof(struct ml_cpu_context);
+
+	struct ml_cpu_context *ctx = (struct ml_cpu_context *)(*kernel_sp);
+	memcpy(ctx, src_ctx, sizeof *ctx);
+
+	ctx->rax = return_value;
+
+	return KERN_OK;
+}
+
 kern_status_t ml_thread_config_get(
 	struct thread *thread,
 	kern_config_key_t key,

include/kernel/thread.h

@@ -62,6 +62,10 @@ extern kern_status_t thread_init_user(
 	virt_addr_t sp,
 	const uintptr_t *args,
 	size_t nr_args);
+extern kern_status_t thread_init_user_clone(
+	struct thread *thr,
+	const struct thread *src,
+	uintptr_t return_value);
 extern int thread_priority(struct thread *thr);
 extern void thread_awaken(struct thread *thr);
 extern void thread_exit(void);

sched/thread.c

@@ -101,6 +101,47 @@ kern_status_t thread_init_user(
 	return KERN_OK;
 }
 
+kern_status_t thread_init_user_clone(
+	struct thread *thr,
+	const struct thread *src,
+	uintptr_t return_value)
+{
+	thr->tr_state = THREAD_READY;
+	thr->tr_quantum_target = default_quantum();
+
+	thr->tr_kstack = vm_page_alloc(THREAD_KSTACK_ORDER, VM_NORMAL);
+	if (!thr->tr_kstack) {
+		return KERN_NO_MEMORY;
+	}
+
+	thr->tr_sp = (uintptr_t)vm_page_get_vaddr(thr->tr_kstack)
+	           + vm_page_order_to_bytes(THREAD_KSTACK_ORDER);
+	thr->tr_bp = thr->tr_sp;
+	thr->tr_cpu_kernel_sp = thr->tr_sp;
+
+	/* the new thread needs two contextx:
+	 *   1) to get the thread running in kernel mode, so that it can
+	 *   execute ml_thread_switch_user
+	 *   2) to allow ml_thread_switch_user to jump to the correct place
+	 *   in usermode (and with the correct stack).
+	 *
+	 * these two contexts are constructed on the thread's kernel stack
+	 * in reverse order.
+	 */
+
+	/* this context will be used by ml_user_return to jump to userspace
+	 * with the specified instruction pointer and user stack */
+	ml_thread_clone_user_context(src->tr_irqctx, return_value, &thr->tr_sp);
+	/* this context will be used by the scheduler and ml_thread_switch to
+	 * jump to ml_user_return in kernel mode with the thread's kernel stack.
+	 */
+	ml_thread_prepare_kernel_context(
+		(uintptr_t)ml_thread_switch_user,
+		&thr->tr_sp);
+
+	return KERN_OK;
+}
+
 void thread_free(struct thread *thr)
 {
 }