New UTCB implementation almost working.

- KIP's pointer to UTCB seems to work with existing l4lib ipc functions.
- Works up to clone()
- In clone we mmap() the same UTCB on each new thread - excessive.
- Generally during page fault handling, cloned threads may fault on the same page
  multiple times even though a single handling would be enough for all of them.
  Need to detect and handle this.

src/api/syscall.c

@@ -79,12 +79,8 @@ void do_exchange_registers(struct ktcb *task, struct exregs_data *exregs)
 		task->pagerid = exregs->pagerid;
 
 	/* Set thread's utcb if supplied */
-	if (exregs->flags & EXREGS_SET_UTCB) {
-		BUG(); /* Check that physical and virtual addresses are in range */
-		task->utcb_phys = exregs->utcb_phys;
-		task->utcb_virt = exregs->utcb_virt;
-	}
-
+	if (exregs->flags & EXREGS_SET_UTCB)
+		task->utcb_address = exregs->utcb_address;
 }
 
 /*
@@ -139,6 +135,12 @@ int sys_exchange_registers(syscall_context_t *regs)
 	}
 #endif
 
+	/* Check UTCB is in valid range */
+	if (exregs->flags & EXREGS_SET_UTCB &&
+	    !(exregs->utcb_address >= UTCB_AREA_START &&
+	    exregs->utcb_address < UTCB_AREA_END))
+		return -EINVAL;
+
 	/* Copy registers */
 	do_exchange_registers(task, exregs);