[PATCH net v4] virtio-net: avoid unnecessary checksum calculation on guest RX

[PATCH net v4] virtio-net: avoid unnecessary checksum calculation on guest RX

[Jon Kohler]

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: Paolo Abeni <pabeni@redhat.com>
Cc: stable@vger.kernel.org
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Fixes: a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP GSO tunneling.")
Signed-off-by: Jon Kohler <jon@nutanix.com>
---
v3: https://patchwork.kernel.org/project/netdevbpf/patch/20251125222754.1737443-1-jon@nutanix.com/
v2: https://patchwork.kernel.org/project/netdevbpf/patch/20251125211418.1707482-1-jon@nutanix.com/
v3-v4: Collect acked-by (MST, Jason) and cc stable (Jason)
v2-v3: Add net tag (whoops, sorry!)
v1-v2: Add arg to avoid conflict from driver (Paolo) and send to net
       instead of net-next.

 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(-)

diff --git a/drivers/net/tun_vnet.h b/drivers/net/tun_vnet.h
index 81662328b2c7..a5f93b6c4482 100644
--- a/drivers/net/tun_vnet.h
+++ b/drivers/net/tun_vnet.h
@@ -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);
 
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index b0947e15895f..1bb3aeca66c6 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -3344,7 +3344,8 @@ 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)
diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index b673c31569f3..75dabb763c65 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -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)
-- 
2.43.0

Re: [PATCH net v4] virtio-net: avoid unnecessary checksum calculation on guest RX

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Wed, 26 Nov 2025 20:38:36 -0700 you wrote:
> 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.
> 
> [...]

Here is the summary with links:
  - [net,v4] virtio-net: avoid unnecessary checksum calculation on guest RX
    https://git.kernel.org/netdev/net/c/1cd1c472343b

You are awesome, thank you!
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