hyperv: Add processing of MTU reduced by the host

If the host uses packet encapsulation feature, the MTU may be reduced by the
host due to headroom reservation for encapsulation. This patch handles this
new MTU value.
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hyperv/netvsc.c

@@ -440,7 +440,8 @@ static int negotiate_nvsp_ver(struct hv_device *device,
 	/* NVSPv2 only: Send NDIS config */
 	memset(init_packet, 0, sizeof(struct nvsp_message));
 	init_packet->hdr.msg_type = NVSP_MSG2_TYPE_SEND_NDIS_CONFIG;
-	init_packet->msg.v2_msg.send_ndis_config.mtu = net_device->ndev->mtu;
+	init_packet->msg.v2_msg.send_ndis_config.mtu = net_device->ndev->mtu +
+						       ETH_HLEN;
 	init_packet->msg.v2_msg.send_ndis_config.capability.ieee8021q = 1;
 
 	ret = vmbus_sendpacket(device->channel, init_packet,

drivers/net/hyperv/netvsc_drv.c

@@ -699,9 +699,10 @@ static int netvsc_change_mtu(struct net_device *ndev, int mtu)
 		return -ENODEV;
 
 	if (nvdev->nvsp_version >= NVSP_PROTOCOL_VERSION_2)
-		limit = NETVSC_MTU;
+		limit = NETVSC_MTU - ETH_HLEN;
 
-	if (mtu < 68 || mtu > limit)
+	/* Hyper-V hosts don't support MTU < ETH_DATA_LEN (1500) */
+	if (mtu < ETH_DATA_LEN || mtu > limit)
 		return -EINVAL;
 
 	nvdev->start_remove = true;

drivers/net/hyperv/rndis_filter.c

@@ -998,6 +998,7 @@ int rndis_filter_device_add(struct hv_device *dev,
 	int t;
 	struct ndis_recv_scale_cap rsscap;
 	u32 rsscap_size = sizeof(struct ndis_recv_scale_cap);
+	u32 mtu, size;
 
 	rndis_device = get_rndis_device();
 	if (!rndis_device)
@@ -1029,6 +1030,14 @@ int rndis_filter_device_add(struct hv_device *dev,
 		return ret;
 	}
 
+	/* Get the MTU from the host */
+	size = sizeof(u32);
+	ret = rndis_filter_query_device(rndis_device,
+					RNDIS_OID_GEN_MAXIMUM_FRAME_SIZE,
+					&mtu, &size);
+	if (ret == 0 && size == sizeof(u32))
+		net_device->ndev->mtu = mtu;
+
 	/* Get the mac address */
 	ret = rndis_filter_query_device_mac(rndis_device);
 	if (ret != 0) {