netdev - Re: [xdp-hints] [PATCH bpf-next v2 6/8] mlx4: Introduce mlx4_xdp_buff wrapper for xdp

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Date:   Wed, 23 Nov 2022 15:33:47 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Stanislav Fomichev <sdf@...gle.com>, bpf@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org,
        Tariq Toukan <tariqt@...dia.com>,
        David Ahern <dsahern@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Anatoly Burakov <anatoly.burakov@...el.com>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        Maryam Tahhan <mtahhan@...hat.com>, xdp-hints@...-project.net,
        netdev@...r.kernel.org
Subject: Re: [xdp-hints] [PATCH bpf-next v2 6/8] mlx4: Introduce
 mlx4_xdp_buff wrapper for xdp_buff

Stanislav Fomichev <sdf@...gle.com> writes:

> No functional changes. Boilerplate to allow stuffing more data after xdp_buff.
>
> Cc: Tariq Toukan <tariqt@...dia.com>
> Cc: John Fastabend <john.fastabend@...il.com>
> Cc: David Ahern <dsahern@...il.com>
> Cc: Martin KaFai Lau <martin.lau@...ux.dev>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Willem de Bruijn <willemb@...gle.com>
> Cc: Jesper Dangaard Brouer <brouer@...hat.com>
> Cc: Anatoly Burakov <anatoly.burakov@...el.com>
> Cc: Alexander Lobakin <alexandr.lobakin@...el.com>
> Cc: Magnus Karlsson <magnus.karlsson@...il.com>
> Cc: Maryam Tahhan <mtahhan@...hat.com>
> Cc: xdp-hints@...-project.net
> Cc: netdev@...r.kernel.org
> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> ---
>  drivers/net/ethernet/mellanox/mlx4/en_rx.c | 26 +++++++++++++---------
>  1 file changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> index 8f762fc170b3..467356633172 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> @@ -661,17 +661,21 @@ static int check_csum(struct mlx4_cqe *cqe, struct sk_buff *skb, void *va,
>  #define MLX4_CQE_STATUS_IP_ANY (MLX4_CQE_STATUS_IPV4)
>  #endif
>  
> +struct mlx4_xdp_buff {
> +	struct xdp_buff xdp;
> +};

This embedding trick works for drivers that put xdp_buff on the stack,
but mlx5 supports XSK zerocopy, which uses the xsk_buff_pool for
allocating them. This makes it a bit awkward to do the same thing there;
and since it's probably going to be fairly common to do something like
this, how about we just add a 'void *drv_priv' pointer to struct
xdp_buff that the drivers can use? The xdp_buff already takes up a full
cache line anyway, so any data stuffed after it will spill over to a new
one; so I don't think there's much difference performance-wise.

I'll send my patch to add support to mlx5 (using the drv_priv pointer
approach) separately.

-Toke