Re: [PATCH v9 1/2] virtio-net: define UDP tunnel segmentation offload feature

[Paolo Abeni <pabeni@redhat.com>]

On 10/28/24 04:27, Jason Wang wrote:
> As the topic becomes more complicated, I wonder if we can do something
> easier, that is focusing on the one checksum offload first. And I
> would like to check the current Linux behaviour first for non-GSO UDP
> tunnel:
> 
> 1) For TX, as you've mentioned, the csum_start/offset has already
> pointed to the inner header (but is this the innermost?)
> 2) For RX, what header would csum_start/offset points (considering we
> have nested tunnels)
> 
> I'm asking to make sure our proposal doesn't conflict with the
> existing implementation.

In Linux, csum_start/offset, both for rx and tx, points to the innermost
_offloaded_ header.

On RX, since GSO partial does not apply, such innermost offloaded header
correspond the outermost/only header (when no tunnels are configured) or
the 2nd outermost, when tunnels are configured and GRO-ed.

On TX, via GSO partial and local csum offload, the csum_start can, in
theory point to an header with an header with an arbitrary high level,
but currently no driver/tx path allow for GSO partial with more than 1
encapsulation level (may change in the future).

The proposed specification change in v9 for NEED_CSUM matches the above.

Since reaching an agreement WRT DATA_VALID handling looks problematic,
would you consider a virtual meeting to discuss the topic?

Thanks,

Paolo

p.s. to be clear, with 'you' I include both Jason and Willem.