releases/6.17.11/virtio-net-avoid-unnecessary-checksum-calculation-on-guest-rx.patch

From 1cd1c472343b06d6d32038636ce51bfa2251e3cf Mon Sep 17 00:00:00 2001
From: Jon Kohler <jon@nutanix.com>
Date: Tue, 25 Nov 2025 15:27:53 -0700
Subject: virtio-net: avoid unnecessary checksum calculation on guest RX

From: Jon Kohler <jon@nutanix.com>

commit 1cd1c472343b06d6d32038636ce51bfa2251e3cf upstream.

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: stable@vger.kernel.org
Fixes: a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP GSO tunneling.")
Signed-off-by: Jon Kohler <jon@nutanix.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://patch.msgid.link/20251125222754.1737443-1-jon@nutanix.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/net/tun_vnet.h     |    2 +-
 drivers/net/virtio_net.c   |    3 ++-
 include/linux/virtio_net.h |    7 ++++---
 3 files changed, 7 insertions(+), 5 deletions(-)

--- a/drivers/net/tun_vnet.h
+++ b/drivers/net/tun_vnet.h
@@ -244,7 +244,7 @@ tun_vnet_hdr_tnl_from_skb(unsigned int f
 
 	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);
 
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -3340,7 +3340,8 @@ static int xmit_skb(struct send_queue *s
 		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)
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -384,7 +384,8 @@ virtio_net_hdr_tnl_from_skb(const struct
 			    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
 	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)