[kernel] Give threads initial arguments

This commit changes the add_user_thunk to point to a new routine,
initialize_user_cpu, which sets all the registers that were previously
unset when starting a new user thread. The values for rdi and rsi are
popped off the initial stack values that add_user_thunk sets up, so that
user thread procs can take up to two arguments.

To suppor this, j6_thread_create gained two new arguments, which are
passed on to the thread.

This also let me finally get rid of the hack of passing an argument in
rsp when starting init.

src/libraries/j6/j6/thread.hh

@@ -1,5 +1,5 @@
 #pragma once
-/// \file thread.h
+/// \file thread.hh
 /// High level threading interface
 
 // The kernel depends on libj6 for some shared code,
@@ -15,7 +15,7 @@ namespace j6 {
 class thread
 {
 public:
-    using proc = void (*)();
+    using proc = void (*)(void *);
 
     /// Constructor. Create a thread and its stack space, but
     /// do not start executing the thread.
@@ -24,8 +24,9 @@ public:
     thread(proc p, uintptr_t stack_top);
 
     /// Start executing the thread.
-    /// \returns  j6_status_ok if the thread was successfully started.
-    j6_status_t start();
+    /// \arg user  Optional pointer to user data to pass to the thread proc
+    /// \returns   j6_status_ok if the thread was successfully started.
+    j6_status_t start(void *user = nullptr);
 
     /// Wait for the thread to stop executing.
     void join();
@@ -34,6 +35,8 @@ public:
     thread(const thread&) = delete;
 
 private:
+    static void init_proc(thread *t, void *user);
+
     j6_status_t m_status;
     j6_handle_t m_stack;
     j6_handle_t m_thread;

src/libraries/j6/thread.cpp

@@ -31,7 +31,7 @@ thread::thread(thread::proc p, uintptr_t stack_top) :
 }
 
 j6_status_t
-thread::start()
+thread::start(void *user)
 {
     if (m_status != j6_status_ok)
         return m_status;
@@ -39,8 +39,12 @@ thread::start()
     if (m_thread != j6_handle_invalid)
         return j6_err_invalid_arg;
 
+    uint64_t arg0 = reinterpret_cast<uint64_t>(this);
+    uint64_t arg1 = reinterpret_cast<uint64_t>(user);
+
     m_status = j6_thread_create(&m_thread, __handle_self,
-        m_stack_top, reinterpret_cast<uintptr_t>(m_proc));
+        m_stack_top, reinterpret_cast<uintptr_t>(init_proc),
+        arg0, arg1);
 
     return m_status;
 }
@@ -51,6 +55,13 @@ thread::join()
     j6_thread_join(m_thread);
 }
 
+void
+thread::init_proc(thread *t, void *user)
+{
+    t->m_proc(user);
+    j6_thread_exit();
 }
 
+} // namespace j6
+
 #endif // __j6kernel