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

[Dumitru Ceara]

On 9/28/20 3:26 PM, Ferruh Yigit wrote:
> On 9/28/2020 2:10 PM, Ananyev, Konstantin wrote:
>>
>>
>>> -----Original Message-----
>>> From: Ferruh Yigit <ferruh.yigit at intel.com>
>>> Sent: Monday, September 28, 2020 1:43 PM
>>> To: Ananyev, Konstantin <konstantin.ananyev at intel.com>; Dumitru Ceara
>>> <dceara at redhat.com>; dev at dpdk.org
>>> Cc: Richardson, Bruce <bruce.richardson at 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.
> 

OK, I can do that and send a v2 to claim both caps together.

Just so that it's clear to me though, these capabilities will only be
advertised and the current behavior of the ring PMD at tx/rx will remain
unchanged, right?

Thanks,
Dumitru

>>
>>>
>>>>>
>>>>>>
>>>>>> 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?
>>
>