netdev - Re: [PATCH v7 bpf-next 05/10] veth: Handle xdp

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Date:   Thu, 2 Aug 2018 15:53:36 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Toshiaki Makita <toshiaki.makita1@...il.com>
Cc:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        John Fastabend <john.fastabend@...il.com>, brouer@...hat.com
Subject: Re: [PATCH v7 bpf-next 05/10] veth: Handle xdp_frames in xdp napi
 ring

On Thu, 2 Aug 2018 22:17:53 +0900
Toshiaki Makita <toshiaki.makita1@...il.com> wrote:

> On 18/08/02 (木) 20:45, Jesper Dangaard Brouer wrote:
> > On Thu,  2 Aug 2018 19:55:09 +0900
> > Toshiaki Makita <makita.toshiaki@....ntt.co.jp> wrote:
> >   
> >> +	headroom = frame->data - delta - (void *)frame;  
> > 
> > Your calculation of headroom is still adding an assumption that
> > xdp_frame is located in the top of data area, that is unnecessary.
> > 
> > The headroom can be calculated as:
> > 
> >   headroom = sizeof(struct xdp_frame) + frame->headroom - delta;  
> 
> Thanks. But I'm not sure I get what you are requesting.

I'm simply requesting you do not use the (void *)frame pointer address,
to calculate the headroom, as it can be calculated in another way.

> Supposing xdp_frame is not located in the top of data area, what ensures 
> that additional sizeof(struct xdp_frame) can be used?

The calculation in convert_to_xdp_frame() assures this.  If we later
add an xdp_frame that is not located in the top of data area, and want
to change the reserved headroom size, then we deal with it, and update
the code.

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