ibmvnic: Ensure that SCRQ entry reads are correctly ordered

Ensure that received Subordinate Command-Response Queue (SCRQ)
entries are properly read in order by the driver. These queues
are used in the ibmvnic device to process RX buffer and TX completion
descriptors. dma_rmb barriers have been added after checking for a
pending descriptor to ensure the correct descriptor entry is checked
and after reading the SCRQ descriptor to ensure the entire
descriptor is read before processing.

Fixes: 032c5e82 ("Driver for IBM System i/p VNIC protocol")
Signed-off-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/ibm/ibmvnic.c

@@ -2404,6 +2404,12 @@ static int ibmvnic_poll(struct napi_struct *napi, int budget)
 
 		if (!pending_scrq(adapter, adapter->rx_scrq[scrq_num]))
 			break;
+		/* The queue entry at the current index is peeked at above
+		 * to determine that there is a valid descriptor awaiting
+		 * processing. We want to be sure that the current slot
+		 * holds a valid descriptor before reading its contents.
+		 */
+		dma_rmb();
 		next = ibmvnic_next_scrq(adapter, adapter->rx_scrq[scrq_num]);
 		rx_buff =
 		    (struct ibmvnic_rx_buff *)be64_to_cpu(next->
@@ -3113,6 +3119,13 @@ static int ibmvnic_complete_tx(struct ibmvnic_adapter *adapter,
 		unsigned int pool = scrq->pool_index;
 		int num_entries = 0;
 
+		/* The queue entry at the current index is peeked at above
+		 * to determine that there is a valid descriptor awaiting
+		 * processing. We want to be sure that the current slot
+		 * holds a valid descriptor before reading its contents.
+		 */
+		dma_rmb();
+
 		next = ibmvnic_next_scrq(adapter, scrq);
 		for (i = 0; i < next->tx_comp.num_comps; i++) {
 			if (next->tx_comp.rcs[i]) {
@@ -3513,6 +3526,11 @@ static union sub_crq *ibmvnic_next_scrq(struct ibmvnic_adapter *adapter,
 	}
 	spin_unlock_irqrestore(&scrq->lock, flags);
 
+	/* Ensure that the entire buffer descriptor has been
+	 * loaded before reading its contents
+	 */
+	dma_rmb();
+
 	return entry;
 }