Re: [dpdk-dev] [PATCH v3 0/5] A means to negotiate delivery of Rx meta data

[Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>]

On 10/1/21 11:11 AM, Thomas Monjalon wrote:
> 01/10/2021 08:47, Andrew Rybchenko:
>> On 9/30/21 10:30 PM, Ivan Malov wrote:
>>> Hi Thomas,
>>>
>>> On 30/09/2021 19:18, Thomas Monjalon wrote:
>>>> 23/09/2021 13:20, Ivan Malov:
>>>>> In 2019, commit [1] announced changes in DEV_RX_OFFLOAD namespace
>>>>> intending to add new flags, RSS_HASH and FLOW_MARK. Since then,
>>>>> only the former has been added. The problem hasn't been solved.
>>>>> Applications still assume that no efforts are needed to enable
>>>>> flow mark and similar meta data delivery.
>>>>>
>>>>> The team behind net/sfc driver has to take over the efforts since
>>>>> the problem has started impacting us. Riverhead, a cutting edge
>>>>> Xilinx smart NIC family, has two Rx prefix types. Rx meta data
>>>>> is available only from long Rx prefix. Switching between the
>>>>> prefix formats can't happen in started state. Hence, we run
>>>>> into the same problem which [1] was aiming to solve.
>>>>
>>>> Sorry I don't understand what is Rx prefix?
>>>
>>> A small chunk of per-packet metadata in Rx packet buffer preceding the
>>> actual packet data. In terms of mbuf, this could be something lying
>>> before m->data_off.
> 
> I've never seen the word "Rx prefix".

Yes, I agree. The term is vendor-specific.

> In general we talk about mbuf headroom and mbuf metadata,
> the rest being the mbuf payload and mbuf tailroom.
> I guess you mean mbuf metadata in the space of the struct rte_mbuf?

Not exactly. It is rather lower level, but finally yes, it goes
to extra data represented by one or another field in mbuf
structure. Broadly Rx metadata is all per-packet extra
information available in HW and could be delivered to SW:
 - Rx checksum offloads information
 - Rx packet classification
 - RSS hash
 - flow mark/flag
 - flow meta
 - tunnel offload information
 - source e-Switch port

Delivering everything is expensive. That's why we have offload
flags, possibility to reduce required Rx packet classification
etc. Some metadata are not covered yet and the series suggest
an approach how to cover it.

> 
>>>>> Rx meta data (mark, flag, tunnel ID) delivery is not an offload
>>>>> on its own since the corresponding flows must be active to set
>>>>> the data in the first place. Hence, adding offload flags
>>>>> similar to RSS_HASH is not a good idea.
>>>>
>>>> What means "active" here?
>>>
>>> Active = inserted and functional. What this paragraph is trying to say
>>> is that when you enable, say, RSS_HASH, that implies both computation of
>>> the hash and the driver's ability to extract in from packets
>>> ("delivery"). But when it comes to MARK, it's just "delivery". No
>>> "offload" here: the NIC won't set any mark in packets unless you create
>>> a flow rule to make it do so. That's the gist of it.
> 
> OK
> Yes I agree RTE_FLOW_ACTION_TYPE_MARK doesn't need any offload flag.
> Same for RTE_FLOW_ACTION_TYPE_SET_META.
> 
>>>>> Patch [1/5] of this series adds a generic API to let applications
>>>>> negotiate delivery of Rx meta data during initialisation period.
> 
> What is a metadata?

See above.

> Do you mean RTE_FLOW_ITEM_TYPE_META and RTE_FLOW_ITEM_TYPE_MARK?
> Metadata word could cover any field in the mbuf struct so it is vague.

We failed to find better term. Yes, it overlaps with other Rx
features. We can document exceptions and add references to
existing ways to control these exceptions.

If you have idea how to name it, you're welcome.

> 
>>>>> This way, an application knows right from the start which parts
>>>>> of Rx meta data won't be delivered. Hence, no necessity to try
>>>>> inserting flows requesting such data and handle the failures.
>>>>
>>>> Sorry I don't understand the problem you want to solve.
>>>> And sorry for not noticing earlier.
>>>
>>> No worries. *Some* PMDs do not enable delivery of, say, Rx mark with the
>>> packets by default (for performance reasons). If the application tries
>>> to insert a flow with action MARK, the PMD may not be able to enable
>>> delivery of Rx mark without the need to re-start Rx sub-system. And
>>> that's fraught with traffic disruption and similar bad consequences. In
>>> order to address it, we need to let the application express its interest
>>> in receiving mark with packets as early as possible. This way, the PMD
>>> can enable Rx mark delivery in advance. And, as an additional benefit,
>>> the application can learn *from the very beginning* whether it will be
>>> possible to use the feature or not. If this API tells the application
>>> that no mark delivery will be enabled, then the application can just
>>> skip many unnecessary attempts to insert wittingly unsupported flows
>>> during runtime.
> 
> I'm puzzled, because we could have the same reasoning for any offload.
> I don't understand why we are focusing on mark only.
> I would prefer we find a generic solution using the rte_flow API.
> Can we make rte_flow_validate() working before port start?
> If validating a fake rule doesn't make sense,
> why not having a new function accepting a single action as parameter?

IMHO, it will be misuse of the rte_flow_validate(). It will be
complex from application point of view and driver
implementation point of view since most likely implemented in
a absolutely different code branch.
Also what should be checked for tunnel offload?

> 
>> Thomas, if I'm not mistaken, net/mlx5 dv_xmeta_en driver option
>> is vendor-specific way to address the same problem.
> 
> Not exactly, it is configuring the capabilities:
>   +------+-----------+-----------+-------------+-------------+
>   | Mode | ``MARK``  | ``META``  | ``META`` Tx | FDB/Through |
>   +======+===========+===========+=============+=============+
>   | 0    | 24 bits   | 32 bits   | 32 bits     | no          |
>   +------+-----------+-----------+-------------+-------------+
>   | 1    | 24 bits   | vary 0-32 | 32 bits     | yes         |
>   +------+-----------+-----------+-------------+-------------+
>   | 2    | vary 0-24 | 32 bits   | 32 bits     | yes         |
>   +------+-----------+-----------+-------------+-------------+

Sorry, but I don't understand the difference. Negotiate is
exactly about capabilities which we want to use.