hv_netvsc: Fix IP header checksum for coalesced packets

Incoming packets may have IP header checksum verified by the host.
They may not have IP header checksum computed after coalescing.
This patch re-compute the checksum when necessary, otherwise the
packets may be dropped, because Linux network stack always checks it.
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hyperv/netvsc_drv.c

@@ -744,6 +744,14 @@ void netvsc_linkstatus_callback(struct net_device *net,
 	schedule_delayed_work(&ndev_ctx->dwork, 0);
 }
 
+static void netvsc_comp_ipcsum(struct sk_buff *skb)
+{
+	struct iphdr *iph = (struct iphdr *)skb->data;
+
+	iph->check = 0;
+	iph->check = ip_fast_csum(iph, iph->ihl);
+}
+
 static struct sk_buff *netvsc_alloc_recv_skb(struct net_device *net,
 					     struct netvsc_channel *nvchan)
 {
@@ -770,9 +778,17 @@ static struct sk_buff *netvsc_alloc_recv_skb(struct net_device *net,
 	/* skb is already created with CHECKSUM_NONE */
 	skb_checksum_none_assert(skb);
 
-	/*
-	 * In Linux, the IP checksum is always checked.
-	 * Do L4 checksum offload if enabled and present.
+	/* Incoming packets may have IP header checksum verified by the host.
+	 * They may not have IP header checksum computed after coalescing.
+	 * We compute it here if the flags are set, because on Linux, the IP
+	 * checksum is always checked.
+	 */
+	if (csum_info && csum_info->receive.ip_checksum_value_invalid &&
+	    csum_info->receive.ip_checksum_succeeded &&
+	    skb->protocol == htons(ETH_P_IP))
+		netvsc_comp_ipcsum(skb);
+
+	/* Do L4 checksum offload if enabled and present.
 	 */
 	if (csum_info && (net->features & NETIF_F_RXCSUM)) {
 		if (csum_info->receive.tcp_checksum_succeeded ||