netdev - Re: [RFC PATCH bpf-next 12/12] i40e: implement Tx zero-copy

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Date:   Fri, 18 May 2018 06:23:47 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Magnus Karlsson <magnus.karlsson@...il.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <ast@...com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Netdev <netdev@...r.kernel.org>, michael.lundkvist@...csson.com,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "Singhai, Anjali" <anjali.singhai@...el.com>,
        "Zhang, Qi Z" <qi.z.zhang@...el.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>
Subject: Re: [RFC PATCH bpf-next 12/12] i40e: implement Tx zero-copy

2018-05-17 23:31 GMT+02:00 Jesper Dangaard Brouer <brouer@...hat.com>:
>
> On Tue, 15 May 2018 21:06:15 +0200 Björn Töpel <bjorn.topel@...il.com> wrote:
>
>> From: Magnus Karlsson <magnus.karlsson@...el.com>
>>
>> Here, the zero-copy ndo is implemented. As a shortcut, the existing
>> XDP Tx rings are used for zero-copy. This means that and XDP program
>> cannot redirect to an AF_XDP enabled XDP Tx ring.
>
> This "shortcut" is not acceptable, and completely broken.  The
> XDP_REDIRECT queue_index is based on smp_processor_id(), and can easily
> clash with the configured XSK queue_index.  Provided a bit more code
> context below...
>

Yes, and this is the reason we need to go for a solution with
dedicated Tx rings. Again, we chose not to, and simply drops
XDP_REDIRECT where the AF_XDP queue id clashes with the processor id.
The queue id hijacked by AF_XDP's egress side.

> On Tue, 15 May 2018 21:06:15 +0200
> Björn Töpel <bjorn.topel@...il.com> wrote:
>
> int i40e_xdp_xmit(struct net_device *dev, struct xdp_frame *xdpf)
> {
>         struct i40e_netdev_priv *np = netdev_priv(dev);
>         unsigned int queue_index = smp_processor_id();
>         struct i40e_vsi *vsi = np->vsi;
>         int err;
>
>         if (test_bit(__I40E_VSI_DOWN, vsi->state))
>                 return -ENETDOWN;
>
>> @@ -4025,6 +4158,9 @@ int i40e_xdp_xmit(struct net_device *dev, struct xdp_frame *xdpf)
>>       if (!i40e_enabled_xdp_vsi(vsi) || queue_index >= vsi->num_queue_pairs)
>>               return -ENXIO;
>>
>> +     if (vsi->xdp_rings[queue_index]->xsk_umem)
>> +             return -ENXIO;
>> +
>
> Using the sane errno makes this impossible to debug (via the tracepoints).
>

The rationale was that the situation was similar to an incorrectly
configured receiving (from an XDP_REDIRECT perspective) interface.

We'll rework this! Thanks for looking into this, Jesper!


Björn

>>       err = i40e_xmit_xdp_ring(xdpf, vsi->xdp_rings[queue_index]);
>>       if (err != I40E_XDP_TX)
>>               return -ENOSPC;
>> @@ -4048,5 +4184,34 @@ void i40e_xdp_flush(struct net_device *dev)
>>       if (!i40e_enabled_xdp_vsi(vsi) || queue_index >= vsi->num_queue_pairs)
>>               return;
>>
>> +     if (vsi->xdp_rings[queue_index]->xsk_umem)
>> +             return;
>> +
>>       i40e_xdp_ring_update_tail(vsi->xdp_rings[queue_index]);
>>  }
>
>
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer