[dpdk-dev] [PATCH v7] net/virtio: add set_mtu in virtio

[Dey <sodey@sonusnet.com>]

Virtio interfaces do not currently allow the user to specify a particular 
Maximum Transmission Unit (MTU).Consequently, the MTU of Virtio interfaces 
is typically set to the Ethernet default value of 1500.
This is problematic in the case of cloud deployments, in which a specific
(and potentially non-standard) MTU needs to be set by a DHCP server, which 
needs to be honored by all interfaces across the traffic path.To acheive 
this Virtio interfaces should support setting of MTU.
In case when GRE/VXLAN tunneling is used for internal communication, there 
will be an overhead added by the infrastructure in the packet over and 
above the ETHER MTU of 1518. So to take care of this overhead in these 
cases the DHCP server corrects the L3 MTU to 1454. But since virtio 
interfaces was not having the MTU set functionality that MTU sent by the 
DHCP server was ignored and the instance will still send packets with 1500 
MTU which after encapsulation will become more than 1518 and eventually 
gets dropped in the infrastructure. 
By adding an additional 'set_mtu' function to the Virtio driver, we can 
honor the MTU sent by the DHCP server. The dhcp server/controller can 
then leverage this 'set_mtu' functionality to resolve the above 
mentioned issue of packets getting dropped due to incorrect size.


Signed-off-by: Souvik Dey <sodey@sonusnet.com>

---
Changes in v7:
- Replaced the CRC_LEN with the merge rx buf header length.
- Changed the frame_len max validation to VIRTIO_MAX_RX_PKTLEN.
Changes in v6:
- Description of change corrected
- Corrected the identations
- Corrected the subject line too
- The From line was also not correct
- Re-submitting as the below patch was not proper
Changes in v5: 
- Fix log message for out-of-bounds MTU parameter in virtio_mtu_set
- Calculate frame size, based on 'mtu' parameter
- Corrected the upper bound and lower bound checks in virtio_mtu_set
Changes in v4: Incorporated review comments.
Changes in v3: Corrected few style errors as reported by sys-stv.
Changes in v2: Incorporated review comments.

 drivers/net/virtio/virtio_ethdev.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/net/virtio/virtio_ethdev.c b/drivers/net/virtio/virtio_ethdev.c
index 423c597..1dbfea6 100644
--- a/drivers/net/virtio/virtio_ethdev.c
+++ b/drivers/net/virtio/virtio_ethdev.c
@@ -653,12 +653,20 @@ virtio_dev_allmulticast_disable(struct rte_eth_dev *dev)
                PMD_INIT_LOG(ERR, "Failed to disable allmulticast");
 } 

+#define VLAN_TAG_LEN           4    /* 802.3ac tag (not DMA'd) */
+static int  virtio_mtu_set(struct rte_eth_dev *dev, uint16_t mtu)
+{
+	struct virtio_hw *hw = dev->data->dev_private;
+	uint32_t ether_hdr_len = ETHER_HDR_LEN + VLAN_TAG_LEN +
+		hw->vtnet_hdr_size;
+	uint32_t frame_size = mtu + ether_hdr_len;
+
+	if (mtu < ETHER_MIN_MTU || frame_size > VIRTIO_MAX_RX_PKTLEN) {
+		PMD_INIT_LOG(ERR, "MTU should be between %d and %d\n",
+			ETHER_MIN_MTU, VIRTIO_MAX_RX_PKTLEN);
+		return -EINVAL;
+	}
+	return 0;
+}

 /*
  * dev_ops for virtio, bare necessities for basic operation
  */
@@ -677,7 +685,6 @@ static const struct eth_dev_ops virtio_eth_dev_ops = {
 	.allmulticast_enable     = virtio_dev_allmulticast_enable,
 	.allmulticast_disable    = virtio_dev_allmulticast_disable,
+	.mtu_set                 = virtio_mtu_set,
 	.dev_infos_get           = virtio_dev_info_get,
 	.stats_get               = virtio_dev_stats_get,
 	.xstats_get              = virtio_dev_xstats_get,
-- 
2.9.3.windows.1