netdev - Re: [PATCH RFC net] igb: Fix XDP with PTP enabled

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Date:   Tue, 13 Apr 2021 09:34:49 +0200
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     brouer@...hat.com, Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Sven Auhagen <sven.auhagen@...eatech.de>,
        intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH RFC net] igb: Fix XDP with PTP enabled

On Mon Apr 12 2021, Jesper Dangaard Brouer wrote:
> On Mon, 12 Apr 2021 12:17:13 +0200
> Kurt Kanzenbach <kurt@...utronix.de> wrote:
>
>> When using native XDP with the igb driver, the XDP frame data doesn't point to
>> the beginning of the packet. It's off by 16 bytes. Everything works as expected
>> with XDP skb mode.
>> 
>> Actually these 16 bytes are used to store the packet timestamps. Therefore, pull
>> the timestamp before executing any XDP operations and adjust all other code
>> accordingly. The igc driver does it like that as well.
>
> (Cc. Alexander Duyck)

Thanks.

>
> Do we have enough room for the packet page-split tricks when these 16
> bytes are added?

I think so. AFAICT the timestamp header is accounted. There is
IGB_2K_TOO_SMALL_WITH_PADDING. If 2k isn't sufficient, then 3k buffers
are used.

The only thing this patch does, is adjusting the xdp->data pointer
before executing igb_run_xdp() instead of doing it afterwards. So, that
in the eBPF program `data' points to the packet data, and not to the
timestamp.

Thanks,
Kurt

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