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

[Jason Wang]

On Thu, Nov 27, 2025 at 12:12 AM Jon Kohler <jon@xxxxxxxxxxx> wrote:
>
>
>
> > 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?

I think a new version might be better.

Thanks