Re: [dpdk-dev] [PATCH] net/ring: advertise multi segment support.

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

On 9/28/2020 2:10 PM, Ananyev, Konstantin wrote:
> 
> 
>> -----Original Message-----
>> From: Ferruh Yigit <ferruh.yigit@intel.com>
>> Sent: Monday, September 28, 2020 1:43 PM
>> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>; Dumitru Ceara <dceara@redhat.com>; dev@dpdk.org
>> Cc: Richardson, Bruce <bruce.richardson@intel.com>
>> Subject: Re: [dpdk-dev] [PATCH] net/ring: advertise multi segment support.
>>
>> On 9/28/2020 12:00 PM, Ananyev, Konstantin wrote:
>>>> On 9/28/2020 8:31 AM, Dumitru Ceara wrote:
>>>>> On 9/22/20 4:21 PM, Ferruh Yigit wrote:
>>>>>> On 9/18/2020 11:36 AM, Dumitru Ceara wrote:
>>>>>>> Even though ring interfaces don't support any other TX/RX offloads they
>>>>>>> do support sending multi segment packets and this should be advertised
>>>>>>> in order to not break applications that use ring interfaces.
>>>>>>>
>>>>>>
>>>>>> Does ring PMD support sending multi segmented packets?
>>>>>>
>>>>>
>>>>> Yes, sending multi segmented packets works fine with ring PMD.
>>>>>
>>>>
>>>> Define "works fine" :)
>>>>
>>>> All PMDs can put the first mbuf of the chained mbuf to the ring, in that case
>>>> what is the difference between the ones supports 'DEV_TX_OFFLOAD_MULTI_SEGS' and
>>>> the ones doesn't support?
>>>>
>>>> If the traffic is only from ring PMD to ring PMD, you won't recognize the
>>>> difference between segmented or not-segmented mbufs, and it will look like
>>>> segmented packets works fine.
>>>> But if there is other PMDs involved in the forwarding, or if need to process the
>>>> packets, will it still work fine?
>>>>
>>>>>> As far as I can see ring PMD doesn't know about the mbuf segments.
>>>>>>
>>>>>
>>>>> Right, the PMD doesn't care about the mbuf segments but it implicitly
>>>>> supports sending multi segmented packets. From what I see it's actually
>>>>> the case for most of the PMDs, in the sense that most don't even check
>>>>> the DEV_TX_OFFLOAD_MULTI_SEGS flag and if the application sends multi
>>>>> segment packets they are just accepted.
>>>>    >
>>>>
>>>> As far as I can see, if the segmented packets sent, the ring PMD will put the
>>>> first mbuf into the ring without doing anything specific to the next segments.
>>>>
>>>> If the 'DEV_TX_OFFLOAD_MULTI_SEGS' is supported I expect it should detect the
>>>> segmented packets and put each chained mbuf into the separate field in the ring.
>>>
>>> Hmm, wonder why do you think this is necessary?
>>>   From my perspective current behaviour is sufficient for TX-ing multi-seg packets
>>> over the ring.
>>>
>>
>> I was thinking based on what some PMDs already doing, but right ring may not
>> need to do it.
>>
>> Also for the case, one application is sending multi segmented packets to the
>> ring, and other application pulling packets from the ring and sending to a PMD
>> that does NOT support the multi-seg TX. I thought ring PMD claiming the
>> multi-seg Tx support should serialize packets to support this case, but instead
>> ring claiming 'DEV_RX_OFFLOAD_SCATTER' capability can work by pushing the
>> responsibility to the application.
>>
>> So in this case ring should support both 'DEV_TX_OFFLOAD_MULTI_SEGS' &
>> 'DEV_RX_OFFLOAD_SCATTER', what do you think?
> 
> Seems so...
> Another question - should we allow DEV_TX_OFFLOAD_MULTI_SEGS here,
>   if DEV_RX_OFFLOAD_SCATTER was not specified?
> 

I think better to have a new version of the patch to claim both capabilities 
together.

> 
>>
>>>>
>>>>>
>>>>> However, the fact that the ring PMD doesn't advertise this implicit
>>>>> support forces applications that use ring PMD to have a special case for
>>>>> handling ring interfaces. If the ring PMD would advertise
>>>>> DEV_TX_OFFLOAD_MULTI_SEGS this would allow upper layers to be oblivious
>>>>> to the type of underlying interface.
>>>>>
>>>>
>>>> This is not handling the special case for the ring PMD, this is why he have the
>>>> offload capability flag. Application should behave according capability flags,
>>>> not per specific PMD.
>>>>
>>>> Is there any specific usecase you are trying to cover?
>