Re: [PATCH v6 1/2] virtio-net: define UDP tunnel segmentation offload feature

[Paolo Abeni <pabeni@redhat.com>]

On 7/29/24 23:36, Willem de Bruijn wrote:
> On Mon, Jul 29, 2024 at 5:04 PM Willem de Bruijn <willemb@google.com> wrote:
>>
>> On Mon, Jul 29, 2024 at 7:30 AM Paolo Abeni <pabeni@redhat.com> wrote:
>>>
>>> The VIRTIO_NET_HDR_GSO_UDP_TUNNEL is a gso_type flag allowing GSO over
>>> UDP tunnel. It can be negotiated on both the host and guest sides.
>>>
>>> One constraint addressed here is that the virtio side (either device or
>>> driver) receiving a UDP tunneled GSO packet must be able to reconstruct
>>> completely the inner and outer headers offset - to allow for later GSO.
>>>
>>> To accommodate such need, new fields are introduced in the virtio_net
>>> header: outer_th_offset, inner_protocol_type and inner_nh_offset.
>>> They map directly to the corresponding header information. The inner
>>> transport header is implied by the (inner) checksum offload.
>>>
>>> Those fields are required because a virtio device H/W implementation
>>> performing segmentation for UDP tunneled packet will need to touch
>>> the outer transport protocol (for the UDP length filed), the
>>> inner network protocol (for the total length field, in the IPv4
>>> case).
>>>
>>> Note that segmentation will additionally need to touch
>>> the outer network protocol and the inner transport protocol. The first
>>> is implied/easily found with trivial parsing, the latter is identified
>>> by the existing csum_start field.
>>>
>>> Note that there is no concept of UDP tunnel type negotiation (e.g.
>>> vxlan, geneve, vxlan-gpe, etc.), as a virtio device H/W implementation
>>> can perform segmentation for every possible UDP-tunnel given the
>>> specified new fields.
>>> In the reverse direction, if a virtio device H/W implementation receives
>>> some traffic for an unknown or unsupported UDP tunnel, it will simply
>>> not aggregate the wire packet in GSO one.
>>>
>>> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> 
>>> +
>>> +\begin{note}
>>> +The valid complete checksum of the outer UDP header of individual segments
>>> +can be computed by the driver prior to segmentation only if the GSO packet
>>> +size is a multiple of \field{gso_size}.
>>> +\end{note}
>>
>> Might be helpful to explain why. E.g., ", because then all segments
>> have the same size and thus all data included in the outer UDP
>> checksum is the same for every segment. These pre-computed segment
>> length and checksum fields are different from those of the GSO
>> packet."
> 
> Does this part belong in patch 2/2?

Possibly is a bit border-line.
IMHO it could belong to this patch, too. The idea is to specify one of 
the scenarios what will not require checksum offload.

Thanks,

Paolo