amd-xgbe: Add a read memory barrier to Tx/Rx path

Add a read memory barrier to the Tx and Rx paths where the ownership
bit is checked to be sure that all descriptor fields are read after
having read the ownership bit for the descriptor.

This has not been an issue to date, but it's a good safe-guard to have.
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/amd/xgbe/xgbe-dev.c

@@ -1558,6 +1558,9 @@ static int xgbe_dev_read(struct xgbe_channel *channel)
 	if (XGMAC_GET_BITS_LE(rdesc->desc3, RX_NORMAL_DESC3, OWN))
 		return 1;
 
+	/* Make sure descriptor fields are read after reading the OWN bit */
+	rmb();
+
 #ifdef XGMAC_ENABLE_RX_DESC_DUMP
 	xgbe_dump_rx_desc(ring, rdesc, ring->cur);
 #endif

drivers/net/ethernet/amd/xgbe/xgbe-drv.c

@@ -1791,6 +1791,10 @@ static int xgbe_tx_poll(struct xgbe_channel *channel)
 		if (!hw_if->tx_complete(rdesc))
 			break;
 
+		/* Make sure descriptor fields are read after reading the OWN
+		 * bit */
+		rmb();
+
 #ifdef XGMAC_ENABLE_TX_DESC_DUMP
 		xgbe_dump_tx_desc(ring, ring->dirty, 1, 0);
 #endif