virtio-net: avoid unnecessary checksum calculation on guest RX

Commit a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP
GSO tunneling.") inadvertently altered checksum offload behavior
for guests not using UDP GSO tunneling.

Before, tun_put_user called tun_vnet_hdr_from_skb, which passed
has_data_valid = true to virtio_net_hdr_from_skb.

After, tun_put_user began calling tun_vnet_hdr_tnl_from_skb instead,
which passes has_data_valid = false into both call sites.

This caused virtio hdr flags to not include VIRTIO_NET_HDR_F_DATA_VALID
for SKBs where skb->ip_summed == CHECKSUM_UNNECESSARY. As a result,
guests are forced to recalculate checksums unnecessarily.

Restore the previous behavior by ensuring has_data_valid = true is
passed in the !tnl_gso_type case, but only from tun side, as
virtio_net_hdr_tnl_from_skb() is used also by the virtio_net driver,
which in turn must not use VIRTIO_NET_HDR_F_DATA_VALID on tx.

cc: [e-mail]
Fixes: a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP GSO tunneling.")
Signed-off-by: Jon Kohler <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Acked-by: Jason Wang <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/tun_vnet.h

···
 
 	if (virtio_net_hdr_tnl_from_skb(skb, tnl_hdr, has_tnl_offload,
 					tun_vnet_is_little_endian(flags),
-					vlan_hlen)) {
+					vlan_hlen, true)) {
 		struct virtio_net_hdr_v1 *hdr = &tnl_hdr->hash_hdr.hdr;
 		struct skb_shared_info *sinfo = skb_shinfo(skb);
 

drivers/net/virtio_net.c

···
 		hdr = &skb_vnet_common_hdr(skb)->tnl_hdr;
 
 	if (virtio_net_hdr_tnl_from_skb(skb, hdr, vi->tx_tnl,
-					virtio_is_little_endian(vi->vdev), 0))
+					virtio_is_little_endian(vi->vdev), 0,
+					false))
 		return -EPROTO;
 
 	if (vi->mergeable_rx_bufs)

include/linux/virtio_net.h

···
 			    struct virtio_net_hdr_v1_hash_tunnel *vhdr,
 			    bool tnl_hdr_negotiated,
 			    bool little_endian,
-			    int vlan_hlen)
+			    int vlan_hlen,
+			    bool has_data_valid)
 {
 	struct virtio_net_hdr *hdr = (struct virtio_net_hdr *)vhdr;
 	unsigned int inner_nh, outer_th;
···
 	tnl_gso_type = skb_shinfo(skb)->gso_type & (SKB_GSO_UDP_TUNNEL |
 						    SKB_GSO_UDP_TUNNEL_CSUM);
 	if (!tnl_gso_type)
-		return virtio_net_hdr_from_skb(skb, hdr, little_endian, false,
-					       vlan_hlen);
+		return virtio_net_hdr_from_skb(skb, hdr, little_endian,
+					       has_data_valid, vlan_hlen);
 
 	/* Tunnel support not negotiated but skb ask for it. */
 	if (!tnl_hdr_negotiated)