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 12:48 PM, Thomas Monjalon wrote:
> 01/10/2021 10:55, Ivan Malov:
>> On 01/10/2021 11:11, Thomas Monjalon wrote:
>>> 01/10/2021 08:47, Andrew Rybchenko:
>>>> On 9/30/21 10:30 PM, Ivan Malov wrote:
>>>>> On 30/09/2021 19:18, Thomas Monjalon wrote:
>>>>>> 23/09/2021 13:20, Ivan Malov:
>>>>>>> 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?
>>> 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.
>>
>> Metadata here is *any* additional information provided by the NIC for 
>> each received packet. For example, Rx flag, Rx mark, RSS hash, packet 
>> classification info, you name it. I'd like to stress out that the 
>> suggested API comes with flags each of which is crystal clear on what 
>> concrete kind of metadata it covers, eg. Rx mark.
> 
> I missed the flags.
> You mean these 3 flags?

Yes

> +/** The ethdev sees flagged packets if there are flows with action FLAG. */
> +#define RTE_ETH_RX_META_USER_FLAG (UINT64_C(1) << 0)
> +
> +/** The ethdev sees mark IDs in packets if there are flows with action MARK. */
> +#define RTE_ETH_RX_META_USER_MARK (UINT64_C(1) << 1)
> +
> +/** The ethdev detects missed packets if there are "tunnel_set" flows in use. */
> +#define RTE_ETH_RX_META_TUNNEL_ID (UINT64_C(1) << 2)
> 
> It is not crystal clear because it does not reference the API,
> like RTE_FLOW_ACTION_TYPE_MARK.

Thanks, it is easy to fix. Please, note that there is no action
for tunnel ID case.

> And it covers a limited set of metadata.

Yes which are not covered by offloads, packet classification
etc. Anything else?

> Do you intend to extend to all mbuf metadata?

No. It should be discussed case-by-case separately.

> 
>>>>>>> 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.
>>
>> We're not discussing *offloads*. An offload is when NIC *computes 
>> something* and *delivers* it. We are discussing precisely *delivery*.
> 
> OK but still, there are a lot more mbuf metadata delivered.

Yes, and some are not controlled yet early enough, and
we do here.

> 
>>> I don't understand why we are focusing on mark only
>>
>> We are not focusing on mark on purpose. It's just how our discussion 
>> goes. I chose mark (could've chosen flag or anything else) just to show 
>> you an example.
>>
>>> 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?
>>
>> A noble idea, but if we feed the entire flow rule to the driver for 
>> validation, then the driver must not look specifically for actions FLAG 
>> or MARK in it (to enable or disable metadata delivery). This way, the 
>> driver is obliged to also validate match criteria, attributes, etc. And, 
>> if something is unsupported (say, some specific item), the driver will 
>> have to reject the rule as a whole thus leaving the application to join 
>> the dots itself.
>>
>> Say, you ask the driver to validate the following rule:
>> pattern blah-blah-1 / blah-blah-2 / end action flag / end
>> intending to check support for FLAG delivery. Suppose, the driver 
>> doesn't support pattern item "blah-blah-1". It will throw an error right 
>> after seeing this unsupported item and won't even go further to see the 
>> action FLAG. How can application know whether its request for FLAG was 
>> heard or not?
> 
> No, I'm proposing a new function to validate the action alone,
> without any match etc.
> Example:
> 	rte_flow_action_request(RTE_FLOW_ACTION_TYPE_MARK)

When about tunnel ID?

Also negotiation in terms of bitmask natively allows to
provide everything required at once and it simplifies
implementation in the driver. No dependency on order of
checks etc. Also it allows to renegotiate without any
extra API functions.

> 
> 
>> And I'd not bind delivery of metadata to flow API. Consider the 
>> following example. We have a DPDK application sitting at the *host* and 
>> we have a *guest* with its *own* DPDK instance. The guest DPDK has asked 
>> the NIC (by virtue of flow API) to mark all outgoing packets. This 
>> packets reach the *host* DPDK. Say, the host application just wants to 
>> see the marked packets from the guest. Its own, (the host's) use of flow 
>> API is a don't care here. The host doesn't want to mark packets itself, 
>> it wants to see packets marked by the guest.
> 
> It does not make sense to me. We are talking about a DPDK API.
> My concern is to avoid redefining new flags
> while we already have rte_flow actions.

See above.