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

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Date:   Thu, 12 May 2022 21:34:43 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Saeed Mahameed <saeedm@...dia.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        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>,
        Tariq Toukan <tariqt@...dia.com>,
        Leon Romanovsky <leon@...nel.org>
Subject: Re: [PATCH v6 net-next 13/13] mlx5: support BIG TCP packets

On Thu, May 12, 2022 at 9:29 PM Saeed Mahameed <saeedm@...dia.com> wrote:
>
> 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.

We prefer not changing skb->head, this would break tcpdump.

Surely calling skb_cow_head() would incur a cost.

As I suggested, we can respin the series without the mlx5 patch, this
is totally fine for us, if we can avoid missing 5.19 train.