Re: [PATCH v4] ethdev: add special flags when creating async transfer table

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

On 11/9/22 12:03, Thomas Monjalon wrote:
> 09/11/2022 09:53, Andrew Rybchenko:
>> On 11/8/22 18:25, Thomas Monjalon wrote:
>>> 08/11/2022 15:38, Andrew Rybchenko:
>>>> On 11/8/22 16:29, Thomas Monjalon wrote:
>>>>> 08/11/2022 12:47, Andrew Rybchenko:
>>>>>> On 11/8/22 14:39, Andrew Rybchenko wrote:
>>>>>>> On 11/4/22 13:44, Rongwei Liu wrote:
>>>>>>>> diff --git a/lib/ethdev/rte_flow.h b/lib/ethdev/rte_flow.h
>>>>>>>> index 8858b56428..1eab12796f 100644
>>>>>>>> --- a/lib/ethdev/rte_flow.h
>>>>>>>> +++ b/lib/ethdev/rte_flow.h
>>>>>>>> @@ -5186,6 +5186,34 @@ rte_flow_actions_template_destroy(uint16_t
>>>>>>>> port_id,
>>>>>>>>       */
>>>>>>>>      struct rte_flow_template_table;
>>>>>>>> +/**
>>>>>>>> + * @warning
>>>>>>>> + * @b EXPERIMENTAL: this API may change without prior notice.
>>>>>>>> + *
>>>>>>>> + * Special optional flags for template table attribute.
>>>>>>>> + * Each bit stands for a table specialization
>>>>>>>> + * offering a potential optimization at PMD layer.
>>>>>>>> + * PMD can ignore the unsupported bits silently.
>>>>>>>> + */
>>>>>>>> +enum rte_flow_template_table_specialize {
>>>>>>>> +    /**
>>>>>>>> +     * Specialize table for transfer flows which come only from wire.
>>>>>>>> +     * It allows PMD not to allocate resources for non-wire
>>>>>>>> originated traffic.
>>>>>>>> +     * This bit is not a matching criteria, just an optimization hint.
>>>>>>>> +     * Flow rules which match non-wire originated traffic will be missed
>>>>>>>> +     * if the hint is supported.
>>>>>>
>>>>>> Sorry, but if so, the hint changes behavior.
>>>>>
>>>>> Yes the hint may change behaviour.
>>>>>
>>>>>> Let's consider a rule which matches both VF originating and
>>>>>> wire originating traffic. Will the rule be missed (ignored)
>>>>>> regardless if the hint is supported or not?
>>>>>
>>>>> If the hint RTE_FLOW_TRANSFER_WIRE_ORIG is used,
>>>>> the PMD may assume the table won't be used for traffic
>>>>> which is not coming from wire ports.
>>>>> As a consequence, the table may be implemented on the path
>>>>> of wire traffic only.
>>>>> In this case, the traffic coming from virtual ports
>>>>> won't be affected by this table.
>>>>> To answer the question, a rule matching both virtual and wire traffic
>>>>> will be applied in a table affecting only wire traffic,
>>>>> so it will still apply (not completely ignored).
>>>>
>>>> If so, it is not a hint. It becomes matching criteria
>>>> which should be in pattern as we discussed.
>>>
>>> It is not a strict matching because the PMD is free to support it or not.
>>
>> It cannot be optional matching criteria. Matching criteria must
>> be always mandatory. Otherwise application does not know what
>> to expect and behaviour may legitimately vary on different
>> vendors.
> 
> I think you take it in the wrong direction.
> The idea is not to have it as a criteria.
> Let me explain again:
> 
> If an application is using a flow table to manage flows
> which *always* come from the same type of port (wire or virtual),

What does guarantee it? Is it used a jump-table and jump rule
must guarantee it? Or has pattern corresponding unit?

It is very thin ice and I'm ready to bet money that finally
it will be used as a matching criteria intentionally or not
intentionally. Simply because it works as matching criteria
on, for example, Mellanox. I.e. if rules from table with
corresponding hint are programmed to HW which applies these
rules on traffic from wire only - effectively it is a matching
criteria. And it will be used this way. And it will be not
portable to other HW which does not support the hint.
So, we're making an API which is very easy to misuse if not
to say more.

You know better if it is OK or not to rely on liable users
in the case of DPDK.

It would be much safer if we do not rely on application in this
case, introduce a new pattern item to specify origin and
require PMD to check that pattern has either a new pattern item
or corresponding  REPRESENTED_PORT/PORT_REPRESENTOR pattern
item.

I realize that my concerns could be not valid and it is just
a paranoia. Just add your ack and let's move forward.

> then the application can give this information to the driver.
> With this assumption coming from the application,
> the driver may do some optimizations.
> 
> Now about what is explained above:
> If the application gives such a hint
> but does not respect its own assumption,
> then confusion happens.
> 
>