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

[Hyong Youb Kim <hyonkim@cisco.com>]

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.

Thanks.
-Hyong