Re: [dpdk-dev] [PATCH v2] net/enic: add private API to set ingress VLAN rewrite mode

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

On 3/14/2019 10:04 PM, Thomas Monjalon wrote:
> 14/03/2019 03:58, Hyong Youb Kim:
>> On Wed, Mar 13, 2019 at 10:29:53PM +0100, Thomas Monjalon wrote:
>>> 13/03/2019 22:11, John Daley (johndale):
>>>> From: Thomas Monjalon <thomas@monjalon.net>
>>>>> 13/03/2019 19:32, Ferruh Yigit:
>>>>>> On 3/5/2019 7:11 AM, Hyong Youb Kim wrote:
>>>>>>> The driver currently has a devarg to set the rewrite mode during
>>>>>>> init. Some apps want to programatically set it after running
>>>>>>> rte_eal_init() and finding that ports are VIC. Add a private
>>>>>>> function to support such applications.
>>>>>>
>>>>>> It is not good idea to have PMD specific APIs (although we already have
>>>>> some).
>>>>>>
>>>>>> Specific to this case, as far as I can see it is to pass a config
>>>>>> value and do the action related to it, what would you think having a
>>>>>> generic key/value set/get API in ethdev for this? Similar to rawdev
>>>>> get_attr/set_attr [1]?
>>>>>>
>>>>>> My concern is it may turn into something like ioctl with many things
>>>>>> pushed to it, and cause possible duplication ...
>>>>>
>>>>> Yes, it is clearly ioctl style.
>>>>>
>>>>> Please could you explain more what is the rewrite mode?
>>>>> Does it apply to the port or the queue?
>>>>>
>>>> It applies to a port. By default the Cisco VIC VLAN tags every packet on ingress even if they were untagged coming in on the wire. They are tagged with VLAN 0 or a VLAN id programmed into the NIC depending on the configuration. Its part of the original design, to maintain priority bits, ancient history.
>>>>
>>>> Some apps don't like this (VPP) or take a slower path (OVS). Hyong added a ig-vlan-rewrite=untag devarg to disable this (leave untagged/default vlan packets untagged) during rte_eal_init and this is helpful for OVS, but VPP likes to set the rewrite mode after rte_eal_init() and finding the ports are VIC ports. So that is the reasoning behind the private API call.
>>>
>>> It looks like an application will always set this flag or never.
>>> So I don't see the need for an API function.
>>> Why VPP prefers set this flag later?
>>> Would it be better to have some driver-specific flags, no matter the ports?
>>
>> As is, there seem to be two ways apps deal with NIC-specific tweaks/quirks.
>>
>> 1. Leave everything to the user.
>>
>> Let the human user specify NIC-specific settings (e.g. devarg,
>> not-so-standard MTU/MRU, offloads with not-so-uniform behavior). The
>> app simply passes these to DPDK and does nothing else. Devargs are
>> passed to rte_eal_init. Other settings are applied during the
>> configure phase after rte_eal_init.
>>
>> For example, OVS seems to go for a smallest common denominator that
>> works with most NICs out of the box. Otherwiese, it kinda falls into
>> this camp.
>>
>> For a problematic NIC that needs user intervention, troubleshooting
>> goes like this :-)
>> - Install app.
>> - Run with settings that worked on a previous machine.
>> - Some features suddenly do not work.
>> - Google search this and that ("why this does not work on this server?").
>> - Contact vendors.
>> - Find out this NIC has unexpected behavior.
>> - Set devarg, tweak MTU/MRU, ... ("Oh need to set this for ..").
>> - Now the features work.
>>
>> 2. Hide ugly tweaks from the user.
>>
>> VPP falls into this camp. The user specifies BDFs in the config (no
>> devargs). The app calls rte_eal_init(BDFs), iterates through the
>> discovered ports, applies whatever NIC-specific settings necessary
>> during the configure phase (i.e. do this for vendor A NIC, do that for
>> vendor B NIC), and then start the ports.
>>
>> The ingress vlan rewrite mode is devarg now, so is not usable in this
>> model. One way around it is a private API. Driver specific flags
>> during the configure phase would also work fine. Though, enic might be
>> the only user of those flags.
> 
> I think DPDK needs some driver configuration.
> Currently the config is done per device with devargs.
> The next devargs format allow this:
> 	driver=enic,rewrite=on
> and it can be passed to rte_eal_init().
> 
> We did not progress on the implementation of this format in recent months,
> but you are welcome to help!
> Instead of passing devargs in the whitelist/blacklist options,
> we should introduce a new option, like --dev.

But it will be still devarg in new implementation.
I guess for this use case, there is a need to pass this information from an API.
Options can be:
1- PMD specific API
2- Extend ethdev dev_ops for each usecase
3- Have a generic API, as suggested above
4- Extend configure to accept flags

I don't see a winner in above list, each has some problems. Any comment on how
to proceed?