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

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 8/6/2020 5:39 PM, Slava Ovsiienko wrote:
>> -----Original Message-----
>> From: Ferruh Yigit <ferruh.yigit@intel.com>
>> Sent: Thursday, August 6, 2020 19:37
>> To: Slava Ovsiienko <viacheslavo@mellanox.com>; Andrew Rybchenko
>> <arybchenko@solarflare.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 8/6/2020 5:29 PM, Slava Ovsiienko wrote:
>>>> -----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.
>>
>> If you don't know the packet types in advance, how can you use fixed sizes to
>> split a packet? Won't it be like having random parts of packet in each
>> mempool..
> It is per queue configuration. We have the rte_flow engine and can filter out
> the desired packets to the desired queue.

That is what I was trying to say above, intentions is first filter the packets
to a specific queue, later split them into multiple mempools, you said "not
exactly", what is the difference I am missing?

> 
>>
>>>>>>
>>>>>> 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``
>>>>>
>>>
>