linux-kernel - Re: [PATCH net] veth: xdp: use head instead of hard

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Date:   Tue, 31 Mar 2020 08:35:39 +0900
From:   Toshiaki Makita <toshiaki.makita1@...il.com>
To:     Jesper Dangaard Brouer <jbrouer@...hat.com>,
        Mao Wenan <maowenan@...wei.com>
Cc:     davem@...emloft.net, ast@...nel.org, daniel@...earbox.net,
        kuba@...nel.org, hawk@...nel.org, john.fastabend@...il.com,
        kafai@...com, songliubraving@...com, yhs@...com, andriin@...com,
        jwi@...ux.ibm.com, jianglidong3@...com, edumazet@...gle.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH net] veth: xdp: use head instead of hard_start

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@...wei.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