ibmvnic: add memory barrier to protect long term buffer

dma_rmb() barrier is added to load the long term buffer before copying
it to socket buffer; and dma_wmb() barrier is added to update the
long term buffer before it being accessed by VIOS (virtual i/o server).

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

drivers/net/ethernet/ibm/ibmvnic.c

@@ -1700,6 +1700,9 @@ static netdev_tx_t ibmvnic_xmit(struct sk_buff *skb, struct net_device *netdev)
 		skb_copy_from_linear_data(skb, dst, skb->len);
 	}
 
+	/* post changes to long_term_buff *dst before VIOS accessing it */
+	dma_wmb();
+
 	tx_pool->consumer_index =
 	    (tx_pool->consumer_index + 1) % tx_pool->num_buffers;
 
@@ -2539,6 +2542,8 @@ static int ibmvnic_poll(struct napi_struct *napi, int budget)
 		offset = be16_to_cpu(next->rx_comp.off_frame_data);
 		flags = next->rx_comp.flags;
 		skb = rx_buff->skb;
+		/* load long_term_buff before copying to skb */
+		dma_rmb();
 		skb_copy_to_linear_data(skb, rx_buff->data + offset,
 					length);