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

[Ivan Malov]

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.

>> 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.

>> Patch [1/5] of this series adds a generic API to let applications
>> negotiate delivery of Rx meta data during initialisation period.
>> 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.

-- 
Ivan M