Re: [dpdk-dev] [PATCH v4 1/3] ethdev: support API to set max LRO packet size

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

On 11/8/2019 10:10 AM, Matan Azrad wrote:
> 
> 
> From: Ferruh Yigit
>> On 11/8/2019 6:54 AM, Matan Azrad wrote:
>>> Hi
>>>
>>> From: Ferruh Yigit
>>>> On 11/7/2019 12:35 PM, Dekel Peled wrote:
>>>>> @@ -1266,6 +1286,18 @@ struct rte_eth_dev *
>>>>>
>>>> 	RTE_ETHER_MAX_LEN;
>>>>>  	}
>>>>>
>>>>> +	/*
>>>>> +	 * If LRO is enabled, check that the maximum aggregated packet
>>>>> +	 * size is supported by the configured device.
>>>>> +	 */
>>>>> +	if (dev_conf->rxmode.offloads & DEV_RX_OFFLOAD_TCP_LRO) {
>>>>> +		ret = check_lro_pkt_size(
>>>>> +				port_id, dev_conf-
>>>>> rxmode.max_lro_pkt_size,
>>>>> +				dev_info.max_lro_pkt_size);
>>>>> +		if (ret != 0)
>>>>> +			goto rollback;
>>>>> +	}
>>>>> +
>>>>
>>>> This check forces applications that enable LRO to provide
>> 'max_lro_pkt_size'
>>>> config value.
>>>
>>> Yes.(we can break an API, we noticed it)
>>
>> I am not talking about API/ABI breakage, that part is OK.
>> With this check, if the application requested LRO offload but not provided
>> 'max_lro_pkt_size' value, device configuration will fail.
>>
> Yes
>> Can there be a case application is good with whatever the PMD can support
>> as max?
> Yes can be - you know, we can do everything we want but it is better to be consistent:
> Due to the fact of Max rx pkt len field is mandatory for JUMBO offload, max lro pkt len should be mandatory for LRO offload.
> 
> So your question is actually why both, non-lro packets and LRO packets max size are mandatory...
> 
> 
> I think it should be important values for net applications management.
> Also good for mbuf size managements.
> 
>>>
>>>> - Why it is mandatory now, how it was working before if it is
>>>> mandatory value?
>>>
>>> It is the same as max_rx_pkt_len which is mandatory for jumbo frame
>> offload.
>>> So now, when the user configures a LRO offload he must to set max lro pkt
>> len.
>>> We don't want to confuse the user here with the max rx pkt len
>> configurations and behaviors, they should be with same logic.
>>>
>>> This parameter defines well the LRO behavior.
>>> Before this, each PMD took its own interpretation to what should be the
>> maximum size for LRO aggregated packets.
>>> Now, the user must say what is his intension, and the ethdev can limit it
>> according to the device capability.
>>> By this way, also, the PMD can organize\optimize its data-path more.
>>> Also, the application can create different mempools for LRO queues to
>> allow bigger packet receiving for LRO traffic.
>>>
>>>> - What happens if PMD doesn't provide 'max_lro_pkt_size', so it is '0'?
>>> Yes, you can see the feature description Dekel added.
>>> This patch also updates all the PMDs support an LRO for non-0 value.
>>
>> Of course I can see the updates Matan, my point is "What happens if PMD
>> doesn't provide 'max_lro_pkt_size'",
>> 1) There is no check for it right, so it is acceptable?
> 
> There is check.
> If the capability is 0, any non-zero configuration will fail. 
> 
>> 2) Are we making this filed mandatory to provide for PMDs, it is easy to make
>> new fields mandatory for PMDs but is this really necessary?
> 
> Yes, for consistence.
> 
>>>
>>> as same as max rx pkt len, no?
>>>
>>>> - What do you think setting 'max_lro_pkt_size' config value to what
>>>> PMD provided if application doesn't provide it?
>>> Same answers as above.
>>>
>>
>> If application doesn't care the value, as it has been till now, and not provided
>> explicit 'max_lro_pkt_size', why not ethdev level use the value provided by
>> PMD instead of failing?
> 
> Again, same question we can ask on max rx pkt len.
> 
> Looks like the packet size is very important value which should be set by the application.
> 
> Previous applications have no option to configure it, so they haven't configure it, (probably cover it somehow) I think it is our miss to supply this info.
> 
> Let's do it in same way as we do max rx pkt len (as this patch main idea).
> Later, we can change both to other meaning.
> 

I think it is not a good reason to introduce a new mandatory config option for
application because of 'max_rx_pkt_len' does it.

Will it work, if:
- If application doesn't provide this value, use the PMD max
- If both application and PMD doesn't provide this value, fail on configure()?