[tools] Add j6threads gdb command

The j6threads command shows the current thread, ready threads, and
blocked threads for a given CPU.

To support this, TCB structs gained a pointer to their thread (instead
of trying to do offset magic) and threads gained a pointer to their
creator. Also removed thread::from_tcb() now that the TCB has a pointer.

src/kernel/objects/thread.cpp

@@ -24,12 +24,14 @@ thread::thread(process &parent, uint8_t pri, uintptr_t rsp0) :
 {
     parent.space().initialize_tcb(m_tcb);
     m_tcb.priority = pri;
+    m_tcb.thread = this;
 
     if (!rsp0)
         setup_kernel_stack();
     else
         m_tcb.rsp0 = rsp0;
 
+    m_creator = current_cpu().thread;
     m_self_handle = parent.add_handle(this);
 }
 
@@ -38,14 +40,6 @@ thread::~thread()
     g_kernel_stacks.return_section(m_tcb.kernel_stack);
 }
 
-thread *
-thread::from_tcb(TCB *tcb)
-{
-    static ptrdiff_t offset =
-        -1 * static_cast<ptrdiff_t>(offsetof(thread, m_tcb));
-    return reinterpret_cast<thread*>(util::offset_pointer(tcb, offset));
-}
-
 thread & thread::current() { return *current_cpu().thread; }
 
 inline void schedule_if_current(thread *t) { if (t == current_cpu().thread) scheduler::get().schedule(); }

src/kernel/objects/thread.h

@@ -18,6 +18,9 @@ struct TCB
     uintptr_t rsp3;
     uintptr_t rflags3;
     uintptr_t pml4;
+    // End of area used by asembly
+
+    thread* thread;
 
     uint8_t priority;
     // note: 3 bytes padding
@@ -46,9 +49,6 @@ public:
         none     = 0x00
     };
 
-    /// Get the pointer to the thread object containing this TCB
-    static thread * from_tcb(TCB *tcb);
-
     /// Destructor
     virtual ~thread();
 
@@ -175,6 +175,7 @@ private:
     tcb_node m_tcb;
 
     process &m_parent;
+    thread *m_creator;
 
     state m_state;
     wait_type m_wait_type;