Re: [dpdk-dev] [PATCH] doc: announce changes to ethdev rxconf structure

[Slava Ovsiienko <viacheslavo@mellanox.com>]

> -----Original Message-----
> From: Ferruh Yigit <ferruh.yigit@intel.com>
> Sent: Thursday, August 6, 2020 19:16
> To: Andrew Rybchenko <arybchenko@solarflare.com>; Slava Ovsiienko
> <viacheslavo@mellanox.com>; dev@dpdk.org
> Cc: Matan Azrad <matan@mellanox.com>; Raslan Darawsheh
> <rasland@mellanox.com>; Thomas Monjalon <thomas@monjalon.net>;
> jerinjacobk@gmail.com; stephen@networkplumber.org;
> ajit.khaparde@broadcom.com; maxime.coquelin@redhat.com;
> olivier.matz@6wind.com; david.marchand@redhat.com
> Subject: Re: [PATCH] doc: announce changes to ethdev rxconf structure
> 
> On 8/3/2020 3:31 PM, Andrew Rybchenko wrote:
> > On 8/3/20 1:58 PM, Viacheslav Ovsiienko wrote:
> >> The DPDK datapath in the transmit direction is very flexible.
> >> The applications can build multisegment packets and manages almost
> >> all data aspects - the memory pools where segments are allocated
> >> from, the segment lengths, the memory attributes like external,
> >> registered, etc.
> >>
> >> In the receiving direction, the datapath is much less flexible, the
> >> applications can only specify the memory pool to configure the
> >> receiving queue and nothing more. In order to extend the receiving
> >> datapath capabilities it is proposed to add the new fields into
> >> rte_eth_rxconf structure:
> >>
> >> struct rte_eth_rxconf {
> >>     ...
> >>     uint16_t rx_split_num; /* number of segments to split */
> >>     uint16_t *rx_split_len; /* array of segment lengthes */
> >>     struct rte_mempool **mp; /* array of segment memory pools */
> >>     ...
> >> };
> >>
> >> The non-zero value of rx_split_num field configures the receiving
> >> queue to split ingress packets into multiple segments to the mbufs
> >> allocated from various memory pools according to the specified
> >> lengths. The zero value of rx_split_num field provides the backward
> >> compatibility and queue should be configured in a regular way (with
> >> single/multiple mbufs of the same data buffer length allocated from
> >> the single memory pool).
> >
> > From the above description it is not 100% clear how it will coexist
> > with:
> >  - existing mb_pool argument of the rte_eth_rx_queue_setup()
> 
> +1
- supposed to be NULL if the array of lengths/pools is used

> 
> >  - DEV_RX_OFFLOAD_SCATTER
> >  - DEV_RX_OFFLOAD_HEADER_SPLIT
> > How will application know that the feature is supported? Limitations?
> 
> +1
New flag  DEV_RX_OFFLOAD_BUFFER_SPLIT is supposed to be introduced.
The feature requires the DEV_RX_OFFLOAD_SCATTER is set.
If DEV_RX_OFFLOAD_HEADER_SPLIT is set the error is returned.

> 
> > Is it always split by specified/fixed length?
> > What happens if header length is actually different?
> 
> As far as I understand intention is to filter specific packets to a queue first
> and later do the split, so the header length will be fixed...

Not exactly. The filtering should be handled by rte_flow engine.
The intention is to provide the more flexible way to describe
rx buffers. Currently it is the single pool with fixed size segments. No way to
split the packet into multiple segments with specified lengths and in
the specified pools. What if packet payload should be stored in the physical
memory on other device (GPU/Storage)? What if caching is not desired for
the payload (just forwarding application)? We could provide the special NC pool.
What if packet should be split into the chunks with specific gaps?
For Tx direction we have this opportunity to gather packet from various
pools and any desired combinations , but Rx is much less flexible.
 
> >
> >> The new approach would allow splitting the ingress packets into
> >> multiple parts pushed to the memory with different attributes.
> >> For example, the packet headers can be pushed to the embedded data
> >> buffers within mbufs and the application data into the external
> >> buffers attached to mbufs allocated from the different memory pools.
> >> The memory attributes for the split parts may differ either - for
> >> example the application data may be pushed into the external memory
> >> located on the dedicated physical device, say GPU or NVMe. This would
> >> improve the DPDK receiving datapath flexibility preserving
> >> compatibility with existing API.
> >>
> >> Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
> >> ---
> >>  doc/guides/rel_notes/deprecation.rst | 5 +++++
> >>  1 file changed, 5 insertions(+)
> >>
> >> diff --git a/doc/guides/rel_notes/deprecation.rst
> >> b/doc/guides/rel_notes/deprecation.rst
> >> index ea4cfa7..cd700ae 100644
> >> --- a/doc/guides/rel_notes/deprecation.rst
> >> +++ b/doc/guides/rel_notes/deprecation.rst
> >> @@ -99,6 +99,11 @@ Deprecation Notices
> >>    In 19.11 PMDs will still update the field even when the offload is not
> >>    enabled.
> >>
> >> +* ethdev: add new fields to ``rte_eth_rxconf`` to configure the
> >> +receiving
> >> +  queues to split ingress packets into multiple segments according
> >> +to the
> >> +  specified lengths into the buffers allocated from the specified
> >> +  memory pools. The backward compatibility to existing API is preserved.
> >> +
> >>  * ethdev: ``rx_descriptor_done`` dev_ops and
> ``rte_eth_rx_descriptor_done``
> >>    will be deprecated in 20.11 and will be removed in 21.11.
> >>    Existing ``rte_eth_rx_descriptor_status`` and
> >> ``rte_eth_tx_descriptor_status``
> >