On 19/01/2021 6:36, David Ahern wrote: > On 1/17/21 1:42 AM, Boris Pismenny wrote: >> 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. >> 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.
