On Nov 25, 2025, at 12:57 PM, Paolo Abeni [off-list ref] wrote:

CC netdev

Thats odd, I used git send-email --to-cmd='./scripts/get_maintainer.pl,
but it looks like in MAINTAINERS, that only would have hit
VIRTIO CORE AND NET DRIVERS, which does not include netdev@

Should that have ?
L: netdev@vger.kernel.org <mailto:netdev@vger.kernel.org>

Said another way, should all changes to include/linux/virtio_net.h
be cc’d to netdev DL?

I suspect the answer is yes, I’ll send a patch for that in the
interest of not having this issue again :)

On 11/25/25 6:51 PM, Jon Kohler wrote:

quoted

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.

Cc: Paolo Abeni <pabeni@redhat.com>
Fixes: a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP GSO tunneling.")
Signed-off-by: Jon Kohler <redacted>
---
include/linux/virtio_net.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index b673c31569f3..570c6dd1666d 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h

@@ -394,7 +394,7 @@ 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,
+ return virtio_net_hdr_from_skb(skb, hdr, little_endian, true,
       vlan_hlen);

/* Tunnel support not negotiated but skb ask for it. */

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.

Ah! Good eye, I’ll see what trouble I can get into and send a v2

I think you need to add another argument to
virtio_net_hdr_tnl_from_skb(), or possibly implement a separate helper
to take care of csum offload - the symmetric of
virtio_net_handle_csum_offload().

Also you need to CC netdev, otherwise the patch will not be processed by
patchwork.

/P

No problems on cc’ing netdev, just didn’t realize this one header didn’t
auto cc the list. Will keep an eye on that, and happy to send a patch to
MAINTAINERS file for discussion.