hv_netvsc: Don't ask for additional head room in the skb

The rndis header is 116 bytes big and can be placed in the default
head room that will be available in the skb. Since the netvsc packet
is less than 48 bytes, we can use the skb control buffer
for the netvsc packet. With these changes we don't need to
ask for additional head room.
Signed-off-by: NK. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: NHaiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/hyperv/hyperv_net.h

@@ -124,6 +124,9 @@ struct ndis_tcp_ip_checksum_info;
 /*
  * Represent netvsc packet which contains 1 RNDIS and 1 ethernet frame
  * within the RNDIS
+ *
+ * The size of this structure is less than 48 bytes and we can now
+ * place this structure in the skb->cb field.
  */
 struct hv_netvsc_packet {
 	/* Bookkeeping stuff */

drivers/net/hyperv/netvsc_drv.c

@@ -433,7 +433,6 @@ static int netvsc_start_xmit(struct sk_buff *skb, struct net_device *net)
 	u32 net_trans_info;
 	u32 hash;
 	u32 skb_length;
-	u32 pkt_sz;
 	struct hv_page_buffer page_buf[MAX_PAGE_BUFFER_COUNT];
 	struct netvsc_stats *tx_stats = this_cpu_ptr(net_device_ctx->tx_stats);
 
@@ -461,16 +460,21 @@ static int netvsc_start_xmit(struct sk_buff *skb, struct net_device *net)
 		goto check_size;
 	}
 
-	pkt_sz = sizeof(struct hv_netvsc_packet) + RNDIS_AND_PPI_SIZE;
-
-	ret = skb_cow_head(skb, pkt_sz);
+	/*
+	 * Place the rndis header in the skb head room and
+	 * the skb->cb will be used for hv_netvsc_packet
+	 * structure.
+	 */
+	ret = skb_cow_head(skb, RNDIS_AND_PPI_SIZE);
 	if (ret) {
 		netdev_err(net, "unable to alloc hv_netvsc_packet\n");
 		ret = -ENOMEM;
 		goto drop;
 	}
-	/* Use the headroom for building up the packet */
-	packet = (struct hv_netvsc_packet *)skb->head;
+	/* Use the skb control buffer for building up the packet */
+	BUILD_BUG_ON(sizeof(struct hv_netvsc_packet) >
+			FIELD_SIZEOF(struct sk_buff, cb));
+	packet = (struct hv_netvsc_packet *)skb->cb;
 
 	packet->status = 0;
 	packet->xmit_more = skb->xmit_more;
@@ -483,8 +487,7 @@ static int netvsc_start_xmit(struct sk_buff *skb, struct net_device *net)
 	packet->is_data_pkt = true;
 	packet->total_data_buflen = skb->len;
 
-	rndis_msg = (struct rndis_message *)((unsigned long)packet +
-				sizeof(struct hv_netvsc_packet));
+	rndis_msg = (struct rndis_message *)skb->head;
 
 	memset(rndis_msg, 0, RNDIS_AND_PPI_SIZE);
 
@@ -1118,16 +1121,12 @@ static int netvsc_probe(struct hv_device *dev,
 	struct netvsc_device_info device_info;
 	struct netvsc_device *nvdev;
 	int ret;
-	u32 max_needed_headroom;
 
 	net = alloc_etherdev_mq(sizeof(struct net_device_context),
 				num_online_cpus());
 	if (!net)
 		return -ENOMEM;
 
-	max_needed_headroom = sizeof(struct hv_netvsc_packet) +
-			      RNDIS_AND_PPI_SIZE;
-
 	netif_carrier_off(net);
 
 	net_device_ctx = netdev_priv(net);
@@ -1166,13 +1165,6 @@ static int netvsc_probe(struct hv_device *dev,
 	net->ethtool_ops = &ethtool_ops;
 	SET_NETDEV_DEV(net, &dev->device);
 
-	/*
-	 * Request additional head room in the skb.
-	 * We will use this space to build the rndis
-	 * heaser and other state we need to maintain.
-	 */
-	net->needed_headroom = max_needed_headroom;
-
 	/* Notify the netvsc driver of the new device */
 	memset(&device_info, 0, sizeof(device_info));
 	device_info.ring_size = ring_size;

include/linux/netdevice.h

@@ -132,7 +132,9 @@ static inline bool dev_xmit_complete(int rc)
  *	used.
  */
 
-#if defined(CONFIG_WLAN) || IS_ENABLED(CONFIG_AX25)
+#if defined(CONFIG_HYPERV_NET)
+# define LL_MAX_HEADER 128
+#elif defined(CONFIG_WLAN) || IS_ENABLED(CONFIG_AX25)
 # if defined(CONFIG_MAC80211_MESH)
 #  define LL_MAX_HEADER 128
 # else