Re: [PATCH net v3] virtio-net: avoid unnecessary checksum calculation on guest RX

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Nov 25, 2025 at 03:27:53PM -0700, 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, 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>
> 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>

> ---
> 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 cfa006b88688..96f2d2a59003 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -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)
> 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