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

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Message-ID: <4402804f-bf7b-9ca0-baa9-1415884b0a80@gmail.com>
Date:   Thu, 2 Aug 2018 22:17:53 +0900
From:   Toshiaki Makita <toshiaki.makita1@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
Cc:     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>
Subject: Re: [PATCH v7 bpf-next 05/10] veth: Handle xdp_frames in xdp napi
 ring

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

Toshiaki Makita