上叫
Basically, there is only one upcall, with information as to what events caused it available from a pre_agreed mailbox. 基本上，只有一个回调，带着来自一个预先设定的信箱信息，信息说明在什么事件发生时，线程可用。
The stack on which kse_init() is called must never be unwound, since the kernel fakes up a context at the point where kse_init() is called for each upcall. 调用kse_init()的堆栈永远不能被释放，因为在调用kse_init()的地方，内核为每一个回调伪造了一个上下文。
I'll hang upcall you back, OK? 我先挂了，回头再打给你，好吗？
