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/drivers/net/tun.c b/drivers/net/tun.c
index ef68e13c042d..4dab85eecb60 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -788,6 +788,8 @@ static ssize_t tun_put_user(struct tun_struct *tun,
                         gso.flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
                         gso.csum_start = skb_checksum_start_offset(skb);
                         gso.csum_offset = skb->csum_offset;
+                } else if (skb->ip_summed == CHECKSUM_UNNECESSARY) {
+                        gso.flags = VIRTIO_NET_HDR_F_DATA_VALID;
                 } /* else everything is zero */
 
                 if (unlikely(memcpy_toiovecend(iv, (void *)&gso, total,