Re: [dpdk-dev] [PATCH RFC 00/11] ixgbe/mbuf: add TSO support

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2014-05-09 16:50, Olivier Matz:
> This series add TSO support in ixgbe DPDK driver. As discussed
> previously on the list [1], one problem is that there is not enough room
> in rte_mbuf today to store the required information to implement this
> feature:
>   - a new ol_flag
>   - the MSS
>   - the L4 header len
> 
> A solution would be to increase the size of the mbuf to 2 cache lines
> but it could have a bad impact on performance. This series proposes some
> rework to drastically reduce the size of the rte_mbuf structures before
> implementing the TSO, avoiding to change the mbuf size to 128 bytes.
> 
> After the rework of mbuf structures, the size of rte_mbuf structure is
> reduced by 9 bytes. The implementation of TSO requires to double the
> size of ol_flags (16 to 32 bits) and to double the size of offload
> information in order to add the mss and the l4 header length (32 to 64
> bits). At the end of the whole series, sizeof(rte_mbuf) is still 64
> bytes and 4 bytes are available for future use.
> 
> This rework causes a lot of modifications in the mbuf structure,
> implying some changes in the applications that directly use the mbuf
> structure fields instead of using the API functions (sometimes there is
> no function). That's why this series is a RFC. In my opinion, it's the
> proper moment for this evolution as the 1.7.0 window is open.
> 
> About TSO, the new fields in mbuf try to be generic enough to apply to
> other hardware in the future. To delegate the TCP segmentation to the
> hardware, the user has to:
> 
>   - set the PKT_TX_TCP_SEG flag in mbuf->ol_flags (this flag implies
>     PKT_TX_IP_CKSUM and PKT_TX_TCP_CKSUM)
>   - fill the mbuf->hw_offload information: l2_len, l3_len, l4_len, mss
>   - calculate the pseudo header checksum and set it in the TCP header,
>     as required when doing hardware TCP checksum offload
>   - set the IP checksum to 0

This patchset triggers a lot of interesting discussions about mbuf API, TSO 
and performance impacts.
It seems everybody agree that having TSO support is a nice improvement and 
there are some discussions on how it can be even better.
I feel we need a v2 from Olivier in order to allow us sending improvements 
patches on top of it.

Thanks everyone
-- 
Thomas