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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Wed, 23 Nov 2022 15:33:47 +0100

Stanislav Fomichev <[email-masked]> writes:

> No functional changes. Boilerplate to allow stuffing more data after xdp_buff.
>
> Cc: Tariq Toukan <[email-masked]>
> Cc: John Fastabend <[email-masked]>
> Cc: David Ahern <[email-masked]>
> Cc: Martin KaFai Lau <[email-masked]>
> Cc: Jakub Kicinski <[email-masked]>
> Cc: Willem de Bruijn <[email-masked]>
> Cc: Jesper Dangaard Brouer <[email-masked]>
> Cc: Anatoly Burakov <[email-masked]>
> Cc: Alexander Lobakin <[email-masked]>
> Cc: Magnus Karlsson <[email-masked]>
> Cc: Maryam Tahhan <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Signed-off-by: Stanislav Fomichev <[email-masked]>
> ---
>  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