Extended IPC Support added.

Benefits & Facts:
- Messages up to 2 kilobytes may be sent.
- Both parties may use non-disjoint user buffers. E.g. any userspace address.
- Userspace buffers can page fault.
- Page faults punish timeslice of only the faulting thread.
- Any number of extended ipcs can take place at any one time, since
  only ktcbs of ipc parties are engaged. No global buffer is used.
  - This also provides smp-safety benefit.

Disadvantages:
- There is triple copying penalty. This has to be done:
  - Sender buffer to sender ktcb
  - Sender ktcb to receiver ktcb
  - Receiver ktcb to receiver buffer.
  This is due to the fact that buffers can be on non-disjoint userspace addresses.

If you want to avoid disadvantages and lose some of the benefits,
(e.g. address freedom, shorter copy size) use FULL IPC.

include/l4/generic/tcb.h

@@ -113,9 +113,12 @@ struct ktcb {
 	struct waitqueue_head *waiting_on;
 	struct waitqueue *wq;
 
-	/* Extended ipc buffer, points to the space after ktcb */
-	char *extended_ipc_buffer;
+	/*
+	 * Extended ipc size and buffer that
+	 * points to the space after ktcb
+	 */
 	unsigned long extended_ipc_size;
+	char extended_ipc_buffer[];
 };
 
 /* Per thread kernel stack unified on a single page. */