netdev - Re: [PATCH net] net: cpsw: avoid alignment faults by taking NET_IP

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Message-ID: <CAMj1kXFaM8dH=S3NZw9NqRtAfEa6vMwS0Q0g1v=v9myLyK6VyQ@mail.gmail.com>
Date:   Wed, 19 Jan 2022 16:56:42 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Alexey Smirnov <s.alexey@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Jakub Kicinski <kuba@...nel.org>,
        linux-omap <linux-omap@...r.kernel.org>,
        "open list:BPF JIT for MIPS (32-BIT AND 64-BIT)" 
        <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net: cpsw: avoid alignment faults by taking
 NET_IP_ALIGN into account

On Wed, 19 Jan 2022 at 15:25, Alexey Smirnov <s.alexey@...il.com> wrote:
>
> Doesn't CPSW_HEADROOM already include NET_IP_ALIGN and has actually more strict
> alignment (by sizeof(long)) than CPSW_HEADROOM_NA?

Yes, and that is exactly the problem. NET_IP_ALIGN is used to
deliberately *misalign* the start of the packet in memory, so that the
IP header appears aligned.