Subject: Re: [bpf-next PATCH 2/2] samples/bpf: xdp_rxq_info action XDP_TX must adjust MAC-addrs

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 27 Jun 2018 13:20:26 +0200

On Tue, 26 Jun 2018 17:09:01 -0700
Song Liu <[email-masked]> wrote:

> On Mon, Jun 25, 2018 at 7:27 AM, Jesper Dangaard Brouer
> <[email-masked]> wrote:
> > XDP_TX requires also changing the MAC-addrs, else some hardware
> > may drop the TX packet before reaching the wire.  This was
> > observed with driver mlx5.
> >
> > If xdp_rxq_info select --action XDP_TX the swapmac functionality
> > is activated.  It is also possible to manually enable via cmdline
> > option --swapmac.  This is practical if wanting to measure the
> > overhead of writing/updating payload for other action types.
> >
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
> > ---
[...]
> >
> > diff --git a/samples/bpf/xdp_rxq_info_kern.c b/samples/bpf/xdp_rxq_info_kern.c
> > index 61af6210df2f..222a83eed1cb 100644
> > --- a/samples/bpf/xdp_rxq_info_kern.c
> > +++ b/samples/bpf/xdp_rxq_info_kern.c
> > @@ -21,6 +21,7 @@ struct config {
> >  enum cfg_options_flags {
> >         NO_TOUCH = 0x0U,
> >         READ_MEM = 0x1U,
> > +       SWAP_MAC = 0x2U,
> >  };
[...]
> > @@ -98,7 +116,7 @@ int  xdp_prognum0(struct xdp_md *ctx)
> >                 rxq_rec->issue++;
> >
> >         /* Default: Don't touch packet data, only count packets */
> > -       if (unlikely(config->options & READ_MEM)) {
> > +       if (unlikely(config->options & (READ_MEM|SWAP_MAC))) {
> >                 struct ethhdr *eth = data;
> >
> >                 if (eth + 1 > data_end)
[...]

> > diff --git a/samples/bpf/xdp_rxq_info_user.c b/samples/bpf/xdp_rxq_info_user.c
> > index 435485d4f49e..248a7eab9531 100644
[...]
> > @@ -119,6 +121,8 @@ static char* options2str(enum cfg_options_flags flag)
> >  {
> >         if (flag == NO_TOUCH)
> >                 return "no_touch";
> > +       if (flag & SWAP_MAC)
> > +               return "swapmac";
> >         if (flag & READ_MEM)
> >                 return "read";  
> 
> I guess SWAP_MAC also reads the memory, so it "includes" READ_MEM?

True (see _kern side)

> It is OK for now. We may need to refactor this part when adding other
> flags in the future.

Sure, do remember that this is only a 'sample' program.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer