Re: net/mlx5e: bind() always returns EINVAL with XDP_ZEROCOPY

[Saeed Mahameed <saeedm@mellanox.com>]

On Mon, 2019-09-02 at 11:08 +0200, Jesper Dangaard Brouer wrote:
> On Sun, 1 Sep 2019 18:47:15 +0200
> Kal Cutter Conley <kal.conley@dectris.com> wrote:
> 
> > Hi,
> > I figured out the problem. Let me document the issue here for
> > others
> > and hopefully start a discussion.
> > 
> > The mlx5 driver uses special queue ids for ZC. If N is the number
> > of
> > configured queues, then for XDP_ZEROCOPY the queue ids start at N.
> > So
> > queue ids [0..N) can only be used with XDP_COPY and queue ids
> > [N..2N)
> > can only be used with XDP_ZEROCOPY.
> 
> Thanks for the followup and explanation on how mlx5 AF_XDP queue
> implementation is different from other vendors.
> 
> 
> > sudo ethtool -L eth0 combined 16
> > sudo samples/bpf/xdpsock -r -i eth0 -c -q 0   # OK
> > sudo samples/bpf/xdpsock -r -i eth0 -z -q 0   # ERROR
> > sudo samples/bpf/xdpsock -r -i eth0 -c -q 16  # ERROR
> > sudo samples/bpf/xdpsock -r -i eth0 -z -q 16  # OK
> > 
> > Why was this done? To use zerocopy if available and fallback on
> > copy
> > mode normally you would set sxdp_flags=0. However, here this is no
> > longer possible. To support this driver, you have to first try
> > binding
> > with XDP_ZEROCOPY and the special queue id, then if that fails, you
> > have to try binding again with a normal queue id. Peculiarities
> > like
> > this complicate the XDP user api. Maybe someone can explain the
> > benefits?
> 

in mlx5 we like to keep full functional separation between different
queues. Unlike other implementations in mlx5 kernel standard rx rings
can still function while xsk queues are opened. from user perspective
this should be very simple and very usefull:

queues 0..(N-1): can't be used for XSK ZC since they are standard RX
queues managed by kernel  and driver
queues N..(2N-1): Are XSK user app managed queues, they can't be used
for anything else.

benefits:
- RSS is not interrupted, Ongoing traffic and Current RX queues keeps
going normally when XSK apps are activated/deactivated on the fly.
- Well-defined full logical separation between different types of RX
queue.

as Jesper explained we understand the confusion, and we will come up
with a solution the fits all vendors.

> Thanks for complaining, it is actually valuable. It really illustrate
> the kernel need to improve in this area, which is what our talk[1] at
> LPC2019 (Sep 10) is about.
> 
> Title: "Making Networking Queues a First Class Citizen in the Kernel"
>  [1] https://linuxplumbersconf.org/event/4/contributions/462/
> 
> As you can see, several vendors are actually involved. Kudos to
> Magnus
> for taking initiative here!  It's unfortunately not solved
> "tomorrow",
> as first we have to agree this is needed (facility to register
> queues),
> then agree on API and get commitment from vendors, as this requires
> drivers changes.  There is a long road ahead, but I think it will be
> worthwhile in the end, as effective use of dedicated hardware queues
> (both RX and TX) is key to performance.
>