virtio_net: introduce VIRTIO_NET_HDR_F_DATA_VALID

There's no need for the guest to validate the checksum if it have been
validated by host nics. So this patch introduces a new flag -
VIRTIO_NET_HDR_F_DATA_VALID which is used to bypass the checksum
examing in guest. The backend (tap/macvtap) may set this flag when
met skbs with CHECKSUM_UNNECESSARY to save cpu utilization.

No feature negotiation is needed as old driver just ignore this flag.

Iperf shows 12%-30% performance improvement for UDP traffic. For TCP,
when gro is on no difference as it produces skb with partial
checksum. But when gro is disabled, 20% or even higher improvement
could be measured by netperf.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 67f6749a0a45..b54ec41adea9 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1685,6 +1685,8 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
                         vnet_hdr.flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
                         vnet_hdr.csum_start = skb_checksum_start_offset(skb);
                         vnet_hdr.csum_offset = skb->csum_offset;
+                } else if (skb->ip_summed == CHECKSUM_UNNECESSARY) {
+                        vnet_hdr.flags = VIRTIO_NET_HDR_F_DATA_VALID;
                 } /* else everything is zero */
 
                 err = memcpy_toiovec(msg->msg_iov, (void *)&vnet_hdr,