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

[Paolo Abeni <pabeni@redhat.com>]

CC netdev

On 11/25/25 6:51 PM, Jon Kohler 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.
> 
> 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 <jon@nutanix.com>
> ---
>  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.

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