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

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

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.
> 
>>> 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.
> 

Thomas, if I'm not mistaken, net/mlx5 dv_xmeta_en driver option
is vendor-specific way to address the same problem.