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. */

src/api/ipc.c

@@ -389,34 +389,39 @@ int ipc_recv_extended(l4id_t sendertid, unsigned int flags)
 	/* Obtain extended ipc address */
 	ipc_address = (unsigned long)mr0_current[msg_index];
 
-	/* Set extended ipc buffer as the user buffer address */
-	current->extended_ipc_buffer = (char *)ipc_address;
-
 	/* Obtain extended ipc size */
 	size = extended_ipc_msg_size(flags);
 
 	/* Check size is good */
 	if (size > L4_IPC_EXTENDED_MAX_SIZE)
 		return -EINVAL;
+
+	/* Set extended ipc copy size */
 	current->extended_ipc_size = size;
 
+	/* Engage in real ipc to copy to ktcb buffer */
+	if ((err = ipc_recv(sendertid, flags)) < 0)
+		return err;
+
 	/*
-	 * TODO: We may need to save primary mrs before engaging
-	 * in page fault ipc. Currently after extended ipc address
-	 * is obtained, primaries are not used during the ipc.
-	 * In the future if needed, we should save them.
+	 * NOTE: After this point primary mrs may be trashed.
+	 * We need to save them here if we want to retain their values
+	 * after a page fault ipc. Currently they're not needed.
 	 */
 
-	/* Page fault those pages on the current task if needed */
+	/* Page fault user pages if needed */
 	if ((err = check_access(ipc_address, size,
 				MAP_USR_RW_FLAGS, 1)) < 0)
 		return err;
 
 	/*
-	 * Now we can engage in the real ipc, copying of ipc data
-	 * shall occur during the message copying.
+	 * Now copy from ktcb to user buffers
 	 */
-	return ipc_recv(sendertid, flags);
+	memcpy((void *)ipc_address,
+	       current->extended_ipc_buffer,
+	       current->extended_ipc_size);
+
+	return 0;
 }
 
 
@@ -454,10 +459,9 @@ int ipc_send_extended(l4id_t recv_tid, unsigned int flags)
 	current->extended_ipc_size = size;
 
 	/*
-	 * TODO: We may need to save primary mrs before engaging
-	 * in page fault ipc. Currently after extended ipc address
-	 * is obtained, primaries are not used during the ipc.
-	 * In the future if needed, we should save them.
+	 * NOTE: After this point primary mrs may be trashed.
+	 * We need to save them here if we want to retain their values
+	 * after a page fault ipc. Currently they're not needed.
 	 */
 
 	/* Page fault those pages on the current task if needed */
@@ -465,13 +469,9 @@ int ipc_send_extended(l4id_t recv_tid, unsigned int flags)
 				MAP_USR_RW_FLAGS, 1)) < 0)
 		return err;
 
-	/* Set extended ipc buffer as the end of ktcb */
-	current->extended_ipc_buffer =
-		(void *)current + sizeof(struct ktcb);
-
 	/*
 	 * It is now safe to access user pages.
-	 * Copy message from user buffer into kernel stack
+	 * Copy message from user buffer into current kernel stack
 	 */
 	memcpy(current->extended_ipc_buffer,
 	       (void *)ipc_address, size);

tasks/test0/src/ipctest.c

@@ -89,7 +89,8 @@ void ipc_extended_test(void)
 	 */
 	ipcbuf = base + PAGE_SIZE - L4_IPC_EXTENDED_MAX_SIZE / 2;
 
-	if (!child) {
+	/* Child sending message */
+	if (child == 0) {
 		/* Child creates a string of maximum extended ipc size */
 		for (int i = 0; i < L4_IPC_EXTENDED_MAX_SIZE; i++) {
 			ipcbuf[i] = 'A' + i % 20;
@@ -107,6 +108,7 @@ void ipc_extended_test(void)
 			goto out_err;
 		}
 
+	/* Parent receiving message */
 	} else {
 		/*
 		 * Parent receives on the buffer - we are sure that the
@@ -125,11 +127,16 @@ void ipc_extended_test(void)
 		printf("(%d): Message received from child: %s\n",
 		       getpid(), ipcbuf);
 
-		/* Check that child string is there */
-		for (int i = 0; i < L4_IPC_EXTENDED_MAX_SIZE; i++) {
+		/* Check that string received from child is an exact match */
+		for (int i = 0; i < L4_IPC_EXTENDED_MAX_SIZE - 2; i++) {
 			if (ipcbuf[i] != ('A' + i % 20))
 				goto out_err;
 		}
+		if (ipcbuf[L4_IPC_EXTENDED_MAX_SIZE - 2] != '\n')
+			goto out_err;
+		if (ipcbuf[L4_IPC_EXTENDED_MAX_SIZE - 1] != '\0')
+			goto out_err;
+
 		printf("EXTENDED IPC TEST: -- PASSED --\n");
 	}
 	return;