On 19/01/2021 6:36, David Ahern wrote:

On 1/17/21 1:42 AM, Boris Pismenny wrote:

quoted

This is needed for a few reasons that are explained in detail
in the tcp-ddp offload documentation. See patch 21 overview
and rx-data-path sections. Our reasons are as follows:

I read the documentation patch, and it does not explain it and really
should not since this is very mlx specific based on the changes.
Different h/w will have different limitations. Given that, it would be
best to enhance the patch description to explain why these gymnastics
are needed for the skb.

The text in the documentation that describes this trade-off:
''We remark that a single TCP packet may have numerous PDUs embedded
inside. NICs can choose to offload one or more of these PDUs according
to various trade-offs. Possibly, offloading such small PDUs is of little
value, and it is better to leave it to software. ``

Indeed, different HW may have other additional trade-offs. But, I
suspect that this one will be important for all.

quoted

1) Each SKB may contain multiple PDUs. DDP offload doesn't operate on
PDU headers, so these are written in the receive ring. Therefore, we
need to rebuild the SKB to account for it. Additionally, due to HW
limitations, we will only offload the first PDU in the SKB.

Are you referring to LRO skbs here? I can't imagine going through this
for 1500 byte packets that have multiple PDUs.

No, that is true for any skb, and non-LRO skbs in particular. Most SKBs
do not contain multiple PDUs, but the ones that do are handled
gracefully in this function.



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