Re: [PATCH v2] app/testpmd: use Tx preparation in txonly engine

[Ferruh Yigit <ferruh.yigit@amd.com>]

On 2/8/2024 12:09 PM, Jerin Jacob wrote:
> On Thu, Feb 8, 2024 at 6:15 AM Ferruh Yigit <ferruh.yigit@amd.com> wrote:
>>
>> On 1/11/2024 5:25 AM, Kaiwen Deng wrote:
>>> Txonly forwarding engine does not call the Tx preparation API
>>> before transmitting packets. This may cause some problems.
>>>
>>> TSO breaks when MSS spans more than 8 data fragments. Those
>>> packets will be dropped by Tx preparation API, but it will cause
>>> MDD event if txonly forwarding engine does not call the Tx preparation
>>> API before transmitting packets.
>>>
>>
>> txonly is used commonly, adding Tx prepare for a specific case may
>> impact performance for users.
>>
>> What happens when driver throws MDD (Malicious Driver Detection) event,
>> can't it be ignored? As you are already OK to drop the packet, can
>> device be configured to drop these packages?
>>
>>
>> Or as Jerin suggested adding a new forwarding engine is a solution, but
>> that will create code duplication, I prefer to not have it if this can
> 
> We don't need to have full-blown NEW need forwarding engine.
> Just that, we need to select correct ".packet_fwd" based on the
> offload requirements.
> 
> It is easy to avoid code duplication by following without performance
> impact by moving the logic to compile time and use runtime to fix up
> 
> static inline
> generic_tx_only_packet_forward(...., const unsigned flag)
> {
> 
> #logic common for both packet forward
> #if (flag & NEED_PREPARE)
>    prepare specific code
> 
> }
> 
> static
> generic_tx_only_packet_forward_without_prepare()
> {
> generic_tx_only_packet_forward(..., 0);
> }
> 
> static
> generic_tx_only_packet_forward_with_prepare()
> {
> generic_tx_only_packet_forward(..., NEED_PREPARE);
> }
> 
>  Select the correct .packet_fwd in
> runtime(generic_tx_only_packet_forward_without_prepare() vs
> generic_tx_only_packet_forward_with_prepare())
> 

+1 to not duplicate code, and I guess we can get requested mode as
testpmd parameter, but how can we avoid from performance impact,

checking mode in the .packet_fwd brings additional check per burst, and
we can't overwrite .packet_fwd.