[PATCH 6/6] net/vhost: perform SW checksum in Tx path

[Maxime Coquelin]

Virtio specification supports guest checksum offloading
for L4, which is enabled with VIRTIO_NET_F_GUEST_CSUM
feature negotiation. However, the Vhost PMD does not
advertise Tx checksum offload capabilities.

Advertising these offload capabilities at the ethdev level
is not enough, because we could still end-up with the
application enabling these offloads while the guest not
negotiating it.

This patch advertizes the Tx checksum offload capabilities,
and introduces a compatibility layer to cover the case
VIRTIO_NET_F_GUEST_CSUM has not been negotiated but the
application does configure the Tx checksum offloads. This
function performs the L4 Tx checksum in SW for UDP and TCP.
Compared to Rx SW checksum, the Tx SW checksum function
needs to compute the pseudo-header checksum, as we cannot
knwo whether it was done before.

This patch does not advertize SCTP checksum offloading
capability for now, but it could be handled later if the
need arises.

Reported-by: Jason Wang <jasowang at redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin at redhat.com>
---
 drivers/net/vhost/rte_eth_vhost.c | 62 +++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/drivers/net/vhost/rte_eth_vhost.c b/drivers/net/vhost/rte_eth_vhost.c
index d5303f7368..52a802de05 100644
--- a/drivers/net/vhost/rte_eth_vhost.c
+++ b/drivers/net/vhost/rte_eth_vhost.c
@@ -114,6 +114,7 @@ struct pmd_internal {
 	rte_atomic32_t started;
 	bool vlan_strip;
 	bool rx_sw_csum;
+	bool tx_sw_csum;
 };
 
 struct internal_list {
@@ -370,8 +371,10 @@ vhost_dev_csum_configure(struct rte_eth_dev *eth_dev)
 {
 	struct pmd_internal *internal = eth_dev->data->dev_private;
 	const struct rte_eth_rxmode *rxmode = &eth_dev->data->dev_conf.rxmode;
+	const struct rte_eth_txmode *txmode = &eth_dev->data->dev_conf.txmode;
 
 	internal->rx_sw_csum = false;
+	internal->tx_sw_csum = false;
 
 	/* SW checksum is not compatible with legacy mode */
 	if (!(internal->flags & RTE_VHOST_USER_NET_COMPLIANT_OL_FLAGS))
@@ -384,6 +387,56 @@ vhost_dev_csum_configure(struct rte_eth_dev *eth_dev)
 			internal->rx_sw_csum = true;
 		}
 	}
+
+	if (!(internal->features & (1ULL << VIRTIO_NET_F_GUEST_CSUM))) {
+		if (txmode->offloads &
+				(RTE_ETH_TX_OFFLOAD_UDP_CKSUM | RTE_ETH_TX_OFFLOAD_TCP_CKSUM)) {
+			VHOST_LOG(NOTICE, "Tx csum will be done in SW, may impact performance.");
+			internal->tx_sw_csum = true;
+		}
+	}
+}
+
+static void
+vhost_dev_tx_sw_csum(struct rte_mbuf *mbuf)
+{
+	uint32_t hdr_len;
+	uint16_t csum = 0, csum_offset;
+
+	switch (mbuf->ol_flags & RTE_MBUF_F_TX_L4_MASK) {
+	case RTE_MBUF_F_TX_L4_NO_CKSUM:
+		return;
+	case RTE_MBUF_F_TX_TCP_CKSUM:
+		csum_offset = offsetof(struct rte_tcp_hdr, cksum);
+		break;
+	case RTE_MBUF_F_TX_UDP_CKSUM:
+		csum_offset = offsetof(struct rte_udp_hdr, dgram_cksum);
+		break;
+	default:
+		/* Unsupported packet type. */
+		return;
+	}
+
+	hdr_len = mbuf->l2_len + mbuf->l3_len;
+	csum_offset += hdr_len;
+
+	/* Prepare the pseudo-header checksum */
+	if (rte_net_intel_cksum_prepare(mbuf) < 0)
+		return;
+
+	if (rte_raw_cksum_mbuf(mbuf, hdr_len, rte_pktmbuf_pkt_len(mbuf) - hdr_len, &csum) < 0)
+		return;
+
+	csum = ~csum;
+	/* See RFC768 */
+	if (unlikely((mbuf->packet_type & RTE_PTYPE_L4_UDP) && csum == 0))
+		csum = 0xffff;
+
+	if (rte_pktmbuf_data_len(mbuf) >= csum_offset + 1)
+		*rte_pktmbuf_mtod_offset(mbuf, uint16_t *, csum_offset) = csum;
+
+	mbuf->ol_flags &= ~RTE_MBUF_F_TX_L4_MASK;
+	mbuf->ol_flags |= RTE_MBUF_F_TX_L4_NO_CKSUM;
 }
 
 static void
@@ -513,6 +566,10 @@ eth_vhost_tx(void *q, struct rte_mbuf **bufs, uint16_t nb_bufs)
 			}
 		}
 
+		if (r->internal->tx_sw_csum)
+			vhost_dev_tx_sw_csum(m);
+
+
 		bufs[nb_send] = m;
 		++nb_send;
 	}
@@ -1359,6 +1416,11 @@ eth_dev_info(struct rte_eth_dev *dev,
 
 	dev_info->tx_offload_capa = RTE_ETH_TX_OFFLOAD_MULTI_SEGS |
 				RTE_ETH_TX_OFFLOAD_VLAN_INSERT;
+	if (internal->flags & RTE_VHOST_USER_NET_COMPLIANT_OL_FLAGS) {
+		dev_info->tx_offload_capa |= RTE_ETH_TX_OFFLOAD_UDP_CKSUM |
+			RTE_ETH_TX_OFFLOAD_TCP_CKSUM;
+	}
+
 	dev_info->rx_offload_capa = RTE_ETH_RX_OFFLOAD_VLAN_STRIP;
 	if (internal->flags & RTE_VHOST_USER_NET_COMPLIANT_OL_FLAGS) {
 		dev_info->rx_offload_capa |= RTE_ETH_RX_OFFLOAD_UDP_CKSUM |
-- 
2.35.1