Re: [PATCH net-next v2] netdev: clarify GSO vs csum in qstats

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Jakub Kicinski wrote:
> Could be just me, but I had to pause and double check that the Tx csum
> counter in qstat should include GSO'd packets. GSO pretty much implies
> csum

Unfortunately specifically to virtio_net, this sensible limitation was
not enforced from the start. Which is why virtio_net_hdr_to_skb has a
branch for !VIRTIO_NET_HDR_F_NEEDS_CSUM && gso_type. Mainly "used" by
syzkaller afaik.

With the addition of USO besides TSO that could also eschew L4 checksum
offload. But the local stack does not generate those (udp_send_skb),
nor does UDP GRO (first branch in udp_gro_receive_segment).

In any case, the new comment clearly mentions this limitation on L4
checksum.

> so one could possibly interpret the csum counter as pure csum offload.
> 
> But the counters are based on virtio:
> 
>   [tx_needs_csum]
>       The number of packets which require checksum calculation by the device.
> 
>   [rx_needs_csum]
>       The number of packets with VIRTIO_NET_HDR_F_NEEDS_CSUM.
> 
> and VIRTIO_NET_HDR_F_NEEDS_CSUM gets set on GSO packets virtio sends.
> 
> Clarify this in the spec to avoid any confusion.
> 
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Reviewed-by: Willem de Bruijn <willemb@google.com>

> ---
> v2:
>  - remove the note that almost all GSO types need L4 csum
> v1: https://lore.kernel.org/20250213010457.1351376-1-kuba@kernel.org
> 
> CC: willemb@google.com
> CC: ecree.xilinx@gmail.com
> CC: neescoba@cisco.com
> ---
>  Documentation/netlink/specs/netdev.yaml | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml
> index 288923e965ae..48159eb116a4 100644
> --- a/Documentation/netlink/specs/netdev.yaml
> +++ b/Documentation/netlink/specs/netdev.yaml
> @@ -457,6 +457,8 @@ name: netdev
>          name: tx-needs-csum
>          doc: |
>            Number of packets that required the device to calculate the checksum.
> +          This counter includes the number of GSO wire packets for which device
> +          calculated the L4 checksum.
>          type: uint
>        -
>          name: tx-hw-gso-packets
> -- 
> 2.48.1
>