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

[Jon Kohler]

> On Nov 26, 2025, at 1:29 AM, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> 
> On Wed, Nov 26, 2025 at 5:46 AM Jon Kohler <jon@xxxxxxxxxxx> 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@xxxxxxxxxx>
>> Fixes: a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP GSO tunneling.")
>> Signed-off-by: Jon Kohler <jon@xxxxxxxxxxx>
>> ---
> 
> Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
> 
> (Should this go -stable?)
> 
> Thanks

It could, sure. This made it into 6.17 branch.

Would you like me to send a separate patch with a Cc: stable
or could someone just edit the commit msg when they queue
this?