netdev - Re: net/mlx5e: bind() always returns EINVAL with XDP

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Date:   Thu, 18 Jun 2020 19:31:36 +0200
From:   Kal Cutter Conley <kal.conley@...tris.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        "brouer@...hat.com" <brouer@...hat.com>,
        Maxim Mikityanskiy <maximmi@...lanox.com>,
        "magnus.karlsson@...el.com" <magnus.karlsson@...el.com>,
        "toke.hoiland-jorgensen@....se" <toke.hoiland-jorgensen@....se>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        "gospo@...adcom.com" <gospo@...adcom.com>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bjorn.topel@...el.com" <bjorn.topel@...el.com>
Subject: Re: net/mlx5e: bind() always returns EINVAL with XDP_ZEROCOPY

On Thu, Jun 18, 2020 at 5:23 PM Jonathan Lemon <jonathan.lemon@...il.com> wrote:
>
> On Sun, Jun 14, 2020 at 10:55:30AM +0200, Kal Cutter Conley wrote:
> > Hi Saeed,
> > Thanks for explaining the reasoning behind the special mlx5 queue
> > numbering with XDP zerocopy.
> >
> > We have a process using AF_XDP that also shares the network interface
> > with other processes on the system. ethtool rx flow classification
> > rules are used to route the traffic to the appropriate XSK queue
> > N..(2N-1). The issue is these queues are only valid as long they are
> > active (as far as I can tell). This means if my AF_XDP process dies
> > other processes no longer receive ingress traffic routed over queues
> > N..(2N-1) even though my XDP program is still loaded and would happily
> > always return XDP_PASS. Other drivers do not have this usability issue
> > because they use queues that are always valid. Is there a simple
> > workaround for this issue? It seems to me queues N..(2N-1) should
> > simply map to 0..(N-1) when they are not active?
>
> If your XDP program returns XDP_PASS, the packet should be delivered to
> the xsk socket.  If the application isn't running, where would it go?

XDP_PASS means the packet is passed to the normal network stack for
processing. XDP_REDIRECT means the packet should be delivered to the
xsk socket.

>
> I do agree that the usability of this can be improved.  What if the flow
> rules are inserted and removed along with queue creatioin/destruction?

The problem is the mlx5 driver allows flow rules to be set on
N..(2N-1) at any time; even when no XDP program is loaded. Given this
fact, it would be totally weird if they just suddenly disappeared the
first time the queues go inactive. That's why I suggested that they
just always map to queues 0..(N-1) when they are not active. This way,
at least it's less surprising. What do people think?

> Jonathan

Kal