Re: [dpdk-dev] [PATCH 2/7] ethdev: add mbuf RSS update as a offload

[Andrew Rybchenko <arybchenko@solarflare.com>]

On 8/18/19 7:52 AM, Shahaf Shuler wrote:
> Friday, August 16, 2019 10:48 AM, Andrew Rybchenko:
>> Subject: Re: [dpdk-dev] [PATCH 2/7] ethdev: add mbuf RSS update as a
>> offload
>>
>> On 8/16/19 8:55 AM, pbhagavatula@marvell.com wrote:
>>> From: Pavan Nikhilesh <pbhagavatula@marvell.com>
>>>
>>> Add new Rx offload flag `DEV_RX_OFFLOAD_RSS_HASH` which can be used
>> to
>>> enable/disable PMDs write to `rte_mbuf::hash::rss`.
>> It should be highlighted that presence of the RSS hash is indicated by
>> PKT_RX_RSS_HASH flag in mbuf anyway. Now applications have a way to
>> check that RSS hash delivery is supported and should enable the offload if
>> RSS hash is used. PMD may still provide the hash even if the offload is not
>> enabled.
> I don't understand how PMDs should act w/ this addition when considering the API breakage to application.

There is a deprecation notice for it.
I mentioned in my review notes for one of patches in the series
that the change should be highlighted in release notes.
Yes, it is absolutely required if these patches are accepted.

> Currently application don't set this flag, and expect to get the RSS hash result on mbuf.
> If PMDs will not set the RSS hash result when flag is not present then applications might break.
> If they will always set, then there is no meaning for it.
>
> as I understand the motivation to save few cycles on the PMD receive path, if we want to include it we should treat it as API breakage and documents it on the release notes.
> My option is that some offload should just be usable (OOB) by the fact user enabled them (e.g. RSS). no need to complicate the user by checking and set this field.

What I don't understand is why some offloads should just work but
another requires action to enable it. Just because it is the current
state of things - I don't think it is a good motivation. Sorry.
I think more applications use checksum offloads than RSS hash,
but it is still required to enable it. It looks like no single DPDK example
uses RSS hash. So, I guess it not widely used by applications as well.
Anyway these 2 patches for flow action and RSS hash make all Rx
offloads consistent - if you need something, enable it.

And the question is not to save few cycles in the PMD receive path.
It makes is possible to not deliver both from NIC to host.
8 bytes (4 RSS hash and 4 flow mark) are more than 10% for the
smallest packets.

>>> Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
>> Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>
>>
>> with above and one note below fixed.
>>
>>> ---
>>>    doc/guides/nics/features.rst   | 2 ++
>>>    lib/librte_ethdev/rte_ethdev.h | 1 +
>>>    2 files changed, 3 insertions(+)
>>>
>>> diff --git a/doc/guides/nics/features.rst
>>> b/doc/guides/nics/features.rst index d4d55f721..f79b69b38 100644
>>> --- a/doc/guides/nics/features.rst
>>> +++ b/doc/guides/nics/features.rst
>>> @@ -274,6 +274,7 @@ Supports RSS hashing on RX.
>>>
>>>    * **[uses]     user config**: ``dev_conf.rxmode.mq_mode`` =
>> ``ETH_MQ_RX_RSS_FLAG``.
>>>    * **[uses]     user config**: ``dev_conf.rx_adv_conf.rss_conf``.
>>> +* **[uses]     rte_eth_rxconf,rte_eth_rxmode**:
>> ``offloads:DEV_RX_OFFLOAD_RSS_HASH``.
>>>    * **[provides] rte_eth_dev_info**: ``flow_type_rss_offloads``.
>>>    * **[provides] mbuf**: ``mbuf.ol_flags:PKT_RX_RSS_HASH``, ``mbuf.rss``.
>>>
>>> @@ -286,6 +287,7 @@ Inner RSS
>>>    Supports RX RSS hashing on Inner headers.
>>>
>>>    * **[uses]    rte_flow_action_rss**: ``level``.
>>> +* **[uses]    rte_eth_rxconf,rte_eth_rxmode**:
>> ``offloads:DEV_RX_OFFLOAD_RSS_HASH``.
>>>    * **[provides] mbuf**: ``mbuf.ol_flags:PKT_RX_RSS_HASH``, ``mbuf.rss``.
>>>
>>>
>>> diff --git a/lib/librte_ethdev/rte_ethdev.h
>>> b/lib/librte_ethdev/rte_ethdev.h index f97f0a6e5..889486a11 100644
>>> --- a/lib/librte_ethdev/rte_ethdev.h
>>> +++ b/lib/librte_ethdev/rte_ethdev.h
>>> @@ -1013,6 +1013,7 @@ struct rte_eth_conf {
>>>    #define DEV_RX_OFFLOAD_KEEP_CRC		0x00010000
>>>    #define DEV_RX_OFFLOAD_SCTP_CKSUM	0x00020000
>>>    #define DEV_RX_OFFLOAD_OUTER_UDP_CKSUM  0x00040000
>>> +#define DEV_RX_OFFLOAD_RSS_HASH		0x00080000
>> Should be added to rte_rx_offload_names in
>> lib/librte_ethdev/rte_ethdev.c.