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. > 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. > 2) The newly constructed SKB represents the original data as it is on > the wire, such that the network stack is oblivious to the offload. > 3) We decided not to modify all of the mlx5e_skb_from_cqe* functions > because it would make the offload harder to distinguish, and it would > add overhead to the existing data-path fucntions. Therefore, we opted > for this modular approach. > > If we only had generic header-data split, then we just couldn't > provide this offload. It is not enough to place payload into some > buffer without TCP headers because RPC protocols and advanced storage > protocols, such as nvme-tcp, reorder their responses and require data > to be placed into application/pagecache buffers, which are anything > but anonymous. In other words, header-data split alone writes data > to the wrong buffers (reordering), or to anonymous buffers that > can't be page-flipped to replace application/pagecache buffers. >
