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Subject: Re: [PATCH net] veth: xdp: use head instead of hard_start
To:     Jesper Dangaard Brouer <jbrouer@redhat.com>,
        Mao Wenan <maowenan@huawei.com>
Cc:     davem@davemloft.net, ast@kernel.org, daniel@iogearbox.net,
        kuba@kernel.org, hawk@kernel.org, john.fastabend@gmail.com,
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        jwi@linux.ibm.com, jianglidong3@jd.com, edumazet@google.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        bpf@vger.kernel.org, kernel-janitors@vger.kernel.org
References: <20200330102631.31286-1-maowenan@huawei.com>
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From:   Toshiaki Makita <toshiaki.makita1@gmail.com>
Message-ID: <3053de4c-cee6-f6fc-efc2-09c6250f3ef2@gmail.com>
Date:   Tue, 31 Mar 2020 08:35:39 +0900
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Hi Mao & Jesper
(Resending with plain text...)

On 2020/03/30 20:34, Jesper Dangaard Brouer wrote:
> On Mon, 30 Mar 2020 18:26:31 +0800
> Mao Wenan <maowenan@huawei.com> wrote:
> 
>> xdp.data_hard_start is mapped to the first
>> address of xdp_frame, but the pointer hard_start
>> is the offset(sizeof(struct xdp_frame)) of xdp_frame,
>> it should use head instead of hard_start to
>> set xdp.data_hard_start. Otherwise, if BPF program
>> calls helper_function such as bpf_xdp_adjust_head, it
>> will be confused for xdp_frame_end.
> 
> I have noticed this[1] and have a patch in my current patchset for
> fixing this.  IMHO is is not so important fix right now, as the effect
> is that you currently only lose 32 bytes of headroom.
> 
> [1] https://lore.kernel.org/netdev/158446621887.702578.17234304084556809684.stgit@firesoul/

You are right, the subtraction is not necessary here.
Thank you for working on this.

Toshiaki Makita