Re: [RFC] net/mlx4: add TSO support

["Wiles, Keith" <keith.wiles@intel.com>]

> On May 31, 2018, at 11:21 AM, Moti Haimovsky <motih@mellanox.com> wrote:
> 
> TCP Segmentation Offload (TSO) is a feature which enables the TCP/IP
> network stack to delegate segmentation of a TCP segment to the NIC,
> thus saving compute resources.
> 
> This RFC proposes to add support for TSO to the MLX4 PMD.
> 
> Prerequisites:
> In order for the PMD to recognize the TSO capabilities of the device
> one has to use:
> * RDMA-core v18.0 or above.
> * Linux kernel 4.16 or above.
> 
> Assumptions:
> * mlx4 PMD will follow the TSO support implemented in mlx5 PMD. 
> * PMD is backwards compatible.
>  ** The PMD will continue work with the kernels and RDMA-core
>     supported by it today.
>  ** The PMD will continue to work with devices not supporting TSO. 
> 
> Changes proposed in the PMD for implementing TSO:
> * At init, query the device for TSO support and MAX segment size
>  being supported.
>  This will also determine if the PMD will advertise support for TSO
>  (dev_info->tx_offload_capa |= DEV_TX_OFFLOAD_TCP_TSO;)
> * Calling create-qp when creating a Tx queue will have to consider
>  the MAX TSO header size when calculating the actual queue buffer
>  size. This may be abstracted by calling ibv_create_qp_ex with
>  IBV_QP_INIT_ATTR_MAX_TSO_HEADER as comp flag rather than
>  ibv_create_qp.
>  If this breaks backwards compatibility then this calculation will
>  be done in the PMD code.
> * Modify tx_burst function to:
>  **  Check for TSO flag indication in the packets of the packet burst
>      (buf->ol_flags & PKT_TX_TCP_SEG).
>  **  For TSO packet create the WQE appropriate for sending a TSO packet
>      and fill it with packet info and  L2/L3/L4 Headers.
> * Modify Tx completion function to handle releasing of TSO packet
>  buffers that were transmitted.
> 
> Concerns:
> * Impact of changing Tx send routine on performance.
>  The performance of the tx_burst routine for non-TSO packets may be
>  affected just by placing the code that handles TSO packets in it,
>  so we may want to consider having a dedicated routine for TSO packets.

How much shared code between the two APIs if we created a new API just for TSO?

My first thought was to create a new API, but it would require my application to know it needs to call the new TSO API instead of the normal tx_burst API or does it?
Maybe it does not matter and a TSO request would never be directed to the normal API, if that is the case I would like a new API and not effect the old one. 

> * No MAX-TSO parameter.
>   This is a cross-PMD issue that may need a separate mailing thread to handle.
>   As for today there is no way for the PMD to advertise the MAX-TSO
>   it or its HW support as done with other capabilities.
>   (The indirection table size for example.
>    see rte_eth_dev_info.reta_size in rte_ethdev.h).
>   Also there is no DPDK parameter or constant value that the PMD
>   can use in order to know the MAX-TSO the system requires.
>   This prevents applications from determining the MAX-TSO that can be
>   used leading to configuration mismatches that may lead to transmit
>   failures or to less-than-optimize TSO configuration in the best case.
>   I propose to add a max_tso field in rte_eth_dev_info that will allow
>   the PMD to advertise the max tso is supports. This can be used by
>   DPDK applications to determine what TSO size to use.
>   If this is a major change that cannot fit the 18.08 schedule then
>   I propose to add a MAX_TSO constant in rte_ethdev.h, The PMD will
>   compare this value whit its own MAX-TSO and if it cannot meet the
>   defined value it will not advertise that it is a TSO capable device.
> * Handling packets longer then MAX-TSO
>   In case a PMD is requested to send a TSO packet which is longer than
>   MAX-TSO the PMD send routine should return with an error.
>   A different approach that can be used on the future is to apply GSO
>   to those packets using the GSO lib in DPDK.
> 
> I am interested in general design comments and concerns listed above.
> 
> Signed-off-by: Moti Haimovsky <motih@mellanox.com>
> 
> -- 
> 1.8.3.1
> 

Regards,
Keith