[kernel] Add an IPI to tell a CPU to run the scheduler

When waking another thread, if that thread has a more urgent priority
than the current thread on the same CPU, send that CPU an IPI to tell it
to run its scheduler.

Related changes in this commit:

- Addition of the ipiSchedule isr (vector 0xe4) and its handler in
  isr_handler().
- Change the APIC's send_ipi* functions to take an isr enum and not an
  int for their vector parameter
- Thread TCBs now contain a pointer to their current CPU's cpu_data
  structure
- Add the maybe_schedule() call to the scheduler, which sends the
  schedule IPI to the given thread's CPU only when that CPU is running a
  less-urgent thread.
- Move the locking of a run queue lock earlier in schedule() instead of
  taking the lock in steal_work() and again in schedule().

src/kernel/objects/thread.cpp

@@ -53,7 +53,17 @@ thread::block()
 void
 thread::wake(uint64_t value)
 {
+    if (has_state(state::ready))
+        return;
+
     m_wake_value = value;
+    wake_only();
+    scheduler::get().maybe_schedule(tcb());
+}
+
+void
+thread::wake_only()
+{
     m_wake_timeout = 0;
     set_state(state::ready);
 }

src/kernel/objects/thread.h

@@ -9,6 +9,7 @@
 
 #include "objects/kobject.h"
 
+struct cpu_data;
 struct page_table;
 
 namespace obj {
@@ -37,6 +38,7 @@ struct TCB
     uint64_t last_ran;
 
     uintptr_t kernel_stack;
+    cpu_data *cpu;
 };
 
 using tcb_list = util::linked_list<TCB>;
@@ -45,15 +47,6 @@ using tcb_node = tcb_list::item_type;
 
 namespace obj {
 
-enum class wait_type : uint8_t
-{
-    none   = 0x00,
-    signal = 0x01,
-    time   = 0x02,
-    object = 0x04,
-};
-is_bitfield(wait_type);
-
 class process;
 
 class thread :
@@ -109,6 +102,10 @@ public:
     /// \arg value  The value that block() should return
     void wake(uint64_t value = 0);
 
+    /// Set this thread as awake, but do not call the scheduler
+    /// or set the wake value.
+    void wake_only();
+
     /// Set a timeout to unblock this thread
     /// \arg time  The clock time at which to wake. 0 for no timeout.
     inline void set_wake_timeout(uint64_t time) { m_wake_timeout = time; }