This patch adds ipc_sendrecv() and opens the way for client/server communication.

ipc_sendrecv() replaces ipc_sendwait() which was flawed. See ipc_sendrecv() for
how client/server communication works. Tested with page faults where the kernel
does an ipc_sendrecv() to faulty thread's pager and the pager successfully handles
the request, and returns back the result, which effectively restarts the faulty
thread.

include/l4/api/ipc.h

@@ -16,7 +16,8 @@
 
 #if defined (__KERNEL__) /* These are kernel internal calls */
 /* A helper call for sys_ipc() or internally created ipc paths. */
-int ipc_send(l4id_t tid);
+int ipc_send(l4id_t to);
+int ipc_sendrecv(l4id_t to, l4id_t from);
 
 /*
  * This version sends an extra wait ipc to its receiver so that

loader/start.axf.S

@@ -1,16 +0,0 @@
-
-/*
- * loader/start.axf.S autogenerated from build/start.axf
- *
- * This file is included by the loader sources so that any
- * kernel symbol address can be known in advance and stopped
- * at by debuggers before virtual memory is enabled.
- */
-
-
-.section .text
-.align 4
-.global break_virtual;
-.type	break_virtual, function;
-.equ	break_virtual, 0x106f9c
-

src/api/ipc.c

@@ -154,28 +154,31 @@ int ipc_sendwait(l4id_t to)
 }
 
 /*
- * We currently only support send-receiving from the same task. The receive
- * stage is initiated with the special L4_IPC_TAG_IPCRETURN. This tag is used by
- * client tasks for receiving returned ipc results back. This is by far the most
- * common ipc pattern between client tasks and servers since every such ipc
- * request expects a result.
+ * Both sends and receives mregs in the same call. This is mainly by user
+ * tasks for client server communication with system servers.
+ *
+ * Timeline of client/server communication using ipc_sendrecv():
+ *
+ * (1) User task (client) calls ipc_sendrecv();
+ * (2) System task (server) calls ipc_recv() with from == ANYTHREAD.
+ * (3) Rendezvous occurs. Both tasks exchange mrs and leave rendezvous.
+ * (4,5) User task, immediately calls ipc_recv(), expecting a reply from server.
+ * (4,5) System task handles the request in userspace.
+ * (6) System task calls ipc_send() sending the return result.
+ * (7) Rendezvous occurs. Both tasks exchange mrs and leave rendezvous.
  */
 int ipc_sendrecv(l4id_t to, l4id_t from)
 {
 	int ret = 0;
 
 	if (to == from) {
-
-		/* IPC send request stage */
+		/* Send ipc request */
 		ipc_send(to);
 
 		/*
-		 * IPC result return stage.
-		 *
-		 * If the receiving task is scheduled here, (likely to be a
-		 * server which shouldn't block too long) it would only block
-		 * for a fixed amount of time between these send and receive
-		 * calls.
+		 * Get reply.
+		 * A client would block its server only very briefly
+		 * between these calls.
 		 */
 		ipc_recv(from);
 	} else {

src/arch/arm/exception.c

@@ -59,7 +59,7 @@ void fault_ipc_to_pager(u32 faulty_pc, u32 fsr, u32 far)
 				 offsetof(syscall_args_t, r3));
 
 	/* Send ipc to the task's pager */
-	ipc_sendwait(current->pagerid);
+	ipc_sendrecv(current->pagerid, current->pagerid);
 
 	/*
 	 * Pager is now notified and handling the fault. We now sleep on

tasks/mm0/src/fault.c

@@ -368,10 +368,11 @@ int do_anon_page(struct fault_data *fault)
  *  - page needs write access:
  *     action: read the page in, give write access.
  */
-void do_page_fault(struct fault_data *fault)
+int do_page_fault(struct fault_data *fault)
 {
 	unsigned int vma_flags = (fault->vma) ? fault->vma->flags : VM_NONE;
 	unsigned int reason = fault->reason;
+	int err;
 
 	/* vma flags show no access */
 	if (vma_flags & VM_NONE) {
@@ -394,9 +395,15 @@ void do_page_fault(struct fault_data *fault)
 
 	/* Handle legitimate read faults on the vma */
 	if (vma_flags & VMA_ANON)
-		do_anon_page(fault);
+		err = do_anon_page(fault);
 	else
-		do_file_page(fault);
+		err = do_file_page(fault);
+
+	/* Return the ipc and by doing so restart the faulty thread */
+	printf("Finished handling fault. Restarting thread by returning.\n");
+
+	l4_ipc_return(err);
+	return 0;
 }
 
 void vm_file_pager_read_page(struct fault_data *fault, void *dest_page)