Commit 1cd1c472 authored by Jon Kohler's avatar Jon Kohler Committed by Jakub Kicinski
Browse files

virtio-net: avoid unnecessary checksum calculation on guest RX 



Commit a2fb4bc4 ("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: stable@vger.kernel.org
Fixes: a2fb4bc4 ("net: implement virtio helpers to handle UDP GSO tunneling.")
Signed-off-by: default avatarJon Kohler <jon@nutanix.com>
Acked-by: default avatarMichael S. Tsirkin <mst@redhat.com>
Acked-by: default avatarJason Wang <jasowang@redhat.com>
Link: https://patch.msgid.link/20251125222754.1737443-1-jon@nutanix.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 6d66e093

drivers/net/tun_vnet.h

+1 −1
Original line number Diff line number Diff line
@@ -244,7 +244,7 @@ tun_vnet_hdr_tnl_from_skb(unsigned int flags, 


	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

+2 −1
Original line number Diff line number Diff line
@@ -3339,7 +3339,8 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb, bool orphan) 
		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

+4 −3
Original line number Diff line number Diff line
@@ -384,7 +384,8 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb, 
			    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;
@@ -394,8 +395,8 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb, 
	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)