Index: head/sys/dev/hyperv/netvsc/if_hn.c
===================================================================
--- head/sys/dev/hyperv/netvsc/if_hn.c
+++ head/sys/dev/hyperv/netvsc/if_hn.c
@@ -62,6 +62,7 @@
 
 #include <sys/param.h>
 #include <sys/bus.h>
+#include <sys/counter.h>
 #include <sys/kernel.h>
 #include <sys/limits.h>
 #include <sys/malloc.h>
@@ -460,6 +461,35 @@
     "Trust ip packet verification on host side, "
     "when csum info is missing (global setting)");
 
+/*
+ * Offload UDP/IPv4 checksum.
+ */
+static int			hn_enable_udp4cs = 1;
+SYSCTL_INT(_hw_hn, OID_AUTO, enable_udp4cs, CTLFLAG_RDTUN,
+    &hn_enable_udp4cs, 0, "Offload UDP/IPv4 checksum");
+
+/*
+ * Offload UDP/IPv6 checksum.
+ */
+static int			hn_enable_udp6cs = 1;
+SYSCTL_INT(_hw_hn, OID_AUTO, enable_udp6cs, CTLFLAG_RDTUN,
+    &hn_enable_udp6cs, 0, "Offload UDP/IPv6 checksum");
+
+/* Stats. */
+static counter_u64_t		hn_udpcs_fixup;
+SYSCTL_COUNTER_U64(_hw_hn, OID_AUTO, udpcs_fixup, CTLFLAG_RW,
+    &hn_udpcs_fixup, "# of UDP checksum fixup");
+
+/*
+ * See hn_set_hlen().
+ *
+ * This value is for Azure.  For Hyper-V, set this above
+ * 65536 to disable UDP datagram checksum fixup.
+ */
+static int			hn_udpcs_fixup_mtu = 1420;
+SYSCTL_INT(_hw_hn, OID_AUTO, udpcs_fixup_mtu, CTLFLAG_RWTUN,
+    &hn_udpcs_fixup_mtu, 0, "UDP checksum fixup MTU threshold");
+
 /* Limit TSO burst size */
 static int			hn_tso_maxlen = IP_MAXPACKET;
 SYSCTL_INT(_hw_hn, OID_AUTO, tso_maxlen, CTLFLAG_RDTUN,
@@ -799,6 +829,27 @@
 		ip = mtodo(m_head, ehlen);
 		iphlen = ip->ip_hl << 2;
 		m_head->m_pkthdr.l3hlen = iphlen;
+
+		/*
+		 * UDP checksum offload does not work in Azure, if the
+		 * following conditions meet:
+		 * - sizeof(IP hdr + UDP hdr + payload) > 1420.
+		 * - IP_DF is not set in the IP hdr.
+		 *
+		 * Fallback to software checksum for these UDP datagrams.
+		 */
+		if ((m_head->m_pkthdr.csum_flags & CSUM_IP_UDP) &&
+		    m_head->m_pkthdr.len > hn_udpcs_fixup_mtu + ehlen &&
+		    (ntohs(ip->ip_off) & IP_DF) == 0) {
+			uint16_t off = ehlen + iphlen;
+
+			counter_u64_add(hn_udpcs_fixup, 1);
+			PULLUP_HDR(m_head, off + sizeof(struct udphdr));
+			*(uint16_t *)(m_head->m_data + off +
+                            m_head->m_pkthdr.csum_data) = in_cksum_skip(
+			    m_head, m_head->m_pkthdr.len, off);
+			m_head->m_pkthdr.csum_flags &= ~CSUM_IP_UDP;
+		}
 	}
 #endif
 #if defined(INET6) && defined(INET)
@@ -5479,11 +5530,11 @@
 		csum_assist |= CSUM_IP;
 	if (sc->hn_caps & HN_CAP_TCP4CS)
 		csum_assist |= CSUM_IP_TCP;
-	if (sc->hn_caps & HN_CAP_UDP4CS)
+	if ((sc->hn_caps & HN_CAP_UDP4CS) && hn_enable_udp4cs)
 		csum_assist |= CSUM_IP_UDP;
 	if (sc->hn_caps & HN_CAP_TCP6CS)
 		csum_assist |= CSUM_IP6_TCP;
-	if (sc->hn_caps & HN_CAP_UDP6CS)
+	if ((sc->hn_caps & HN_CAP_UDP6CS) && hn_enable_udp6cs)
 		csum_assist |= CSUM_IP6_UDP;
 	for (i = 0; i < sc->hn_tx_ring_cnt; ++i)
 		sc->hn_tx_ring[i].hn_csum_assist = csum_assist;
@@ -7336,6 +7387,8 @@
 {
 	int i;
 
+	hn_udpcs_fixup = counter_u64_alloc(M_WAITOK);
+
 #ifdef HN_IFSTART_SUPPORT
 	/*
 	 * Don't use ifnet.if_start if transparent VF mode is requested;
@@ -7415,5 +7468,7 @@
 	if (hn_vfmap != NULL)
 		free(hn_vfmap, M_DEVBUF);
 	rm_destroy(&hn_vfmap_lock);
+
+	counter_u64_free(hn_udpcs_fixup);
 }
 SYSUNINIT(hn_sysuninit, SI_SUB_DRIVERS, SI_ORDER_SECOND, hn_sysuninit, NULL);