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 <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/macvtap.c

···
 		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 */
 
 	return 0;

drivers/net/tun.c

···
 			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,

drivers/net/virtio_net.c

···
 					  hdr->hdr.csum_start,
 					  hdr->hdr.csum_offset))
 			goto frame_err;
+	} else if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID) {
+		skb->ip_summed = CHECKSUM_UNNECESSARY;
 	}
 
 	skb->protocol = eth_type_trans(skb, dev);

include/linux/virtio_net.h

···
  * specify GSO or CSUM features, you can simply ignore the header. */
 struct virtio_net_hdr {
 #define VIRTIO_NET_HDR_F_NEEDS_CSUM	1	// Use csum_start, csum_offset
+#define VIRTIO_NET_HDR_F_DATA_VALID	2	// Csum is valid
 	__u8 flags;
 #define VIRTIO_NET_HDR_GSO_NONE		0	// Not a GSO frame
 #define VIRTIO_NET_HDR_GSO_TCPV4	1	// GSO frame, IPv4 TCP (TSO)

net/packet/af_packet.c

···
 			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,