netdev - Re: [PATCH v6 net-next 13/13] mlx5: support BIG TCP packets

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Message-ID: <20220513042955.rnid4776hwp556vr@fedora>
Date:   Thu, 12 May 2022 21:29:55 -0700
From:   Saeed Mahameed <saeedm@...dia.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Alexander Duyck <alexanderduyck@...com>,
        Coco Li <lixiaoyan@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Tariq Toukan <tariqt@...dia.com>,
        Leon Romanovsky <leon@...nel.org>
Subject: Re: [PATCH v6 net-next 13/13] mlx5: support BIG TCP packets

On 12 May 11:02, Paolo Abeni wrote:
>On Thu, 2022-05-12 at 01:40 -0700, Saeed Mahameed wrote:
>> On 09 May 20:32, Eric Dumazet wrote:
>> > From: Coco Li <lixiaoyan@...gle.com>
>> >
>> > mlx5 supports LSOv2.
>> >
>> > IPv6 gro/tcp stacks insert a temporary Hop-by-Hop header
>> > with JUMBO TLV for big packets.
>> >
>> > We need to ignore/skip this HBH header when populating TX descriptor.
>> >
>>
>> Sorry i didn't go through all the documentations or previous discussions,
>> please bare with me, so why not clear HBH just before calling the
>> driver xmit ndo ?
>
>I guess this way is more efficient: the driver copies IP hdr and TCP
>hdr directly in the correct/final location into the tx descriptor,
>otherwise the caller would have to memmove L2/L3 just before the driver
>copies them again.
>>

memmove(sizeof(L2/L3)) is not that bad when done only every 64KB+.
it's going to be hard to repeat this and maintain this across all drivers
only to get this micro optimization that I doubt it will be even measurable.

>> Or if HBH has to stick, 
>
>My understanding is that this is not the case.
>
>Cheers,
>
>Paolo
>