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

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Date:   Wed, 30 Jan 2019 20:08:59 +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 Wed, 30 Jan 2019 19:12:48 +0100
Maciej Fijalkowski <maciejromanfijalkowski@...il.com> wrote:

> On Tue, 29 Jan 2019 12:34:05 +0100
> Jesper Dangaard Brouer <brouer@...hat.com> wrote:
> 
> > 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.
> >   
> I'll post a v5 with libbpf_strerror() usage when bpf_set_link_xdp_fd failed in
> samples but at this point it will only give us a standard "device or resource
> busy" error

That is a good first iteration improvement.  And if QA complains, I can
quickly diagnose and explain the issue, based on this generic message,
without digging further and wasting more time.

> , so if we need some more meaningful message that libbpf will give
> us then I guess we need to define a new libbpf_errno entry (as well as entry in
> libbpf_strerror_table for this new errno value) and set the errno in
> bpf_set_link_xdp_fd in case of a failure?

It likely require more work do provide more meaningful messages, and I
guess it is out-of-scope for your patchset.

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