netdev - Re: [PATCH bpf-next v4 5/7] samples/bpf: Add a "force" flag to XDP samples

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Date:   Tue, 29 Jan 2019 12:34:05 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Maciej Fijalkowski <maciejromanfijalkowski@...il.com>
Cc:     daniel@...earbox.net, ast@...nel.org, netdev@...r.kernel.org,
        jakub.kicinski@...ronome.com, brouer@...hat.com
Subject: Re: [PATCH bpf-next v4 5/7] samples/bpf: Add a "force" flag to XDP
 samples

On Tue, 29 Jan 2019 09:00:00 +0100
Jesper Dangaard Brouer <brouer@...hat.com> wrote:

> On Mon, 28 Jan 2019 20:16:11 +0100
> Maciej Fijalkowski <maciejromanfijalkowski@...il.com> wrote:
> 
> > From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> > 
> > Make xdp samples consistent with iproute2 behavior and set the
> > XDP_FLAGS_UPDATE_IF_NOEXIST by default when setting the xdp program on
> > interface. Provide an option for user to force the program loading,
> > which as a result will not include the mentioned flag in
> > bpf_set_link_xdp_fd call.  
> 
> I like the idea, but what is the error message users get after this
> change?

$ sudo ./xdp1 mlx5p1 &
[1] 9768

$ sudo ./xdp1 mlx5p1 
link set xdp fd failed

This error message is a little too generic to be a good user experience.
The kernel (in dev_change_xdp_fd) will return errno -EBUSY (-16), but
we don't use or report the return value in these sample programs.

If my QA see this error message, I will still get an error report
bugzilla that I need to spend time on investigating.  Can we please
improve this error message? 

If you are really cool you get inspired by (or use) libbpf_strerror()
code avail in tools/lib/bpf/libbpf_errno.c.  Default strerror(EBUSY)
will return "Device or resource busy", but maybe we can do slightly
better and report something more meaningful for this XDP context.

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