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

[Jon Kohler]

> On Nov 25, 2025, at 12:57 PM, Paolo Abeni <pabeni@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx <mailto:netdev@xxxxxxxxxxxxxxx>

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:
>> 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@xxxxxxxxxx>
>> Fixes: a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP GSO tunneling.")
>> Signed-off-by: Jon Kohler <jon@xxxxxxxxxxx>
>> ---
>> 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.