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Date:   Wed, 8 Apr 2020 15:52:26 +0300
From:   Tariq Toukan <ttoukan.linux@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>, sameehj@...zon.com
Cc:     Saeed Mahameed <saeedm@...lanox.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, zorik@...zon.com, akiyano@...zon.com,
        gtzalik@...zon.com,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        David Ahern <dsahern@...il.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>
Subject: Re: [PATCH RFC v2 17/33] mlx5: rx queue setup time determine frame_sz
 for XDP

Hi Jesper,

Thanks for your patch.
Please see feedback below.

On 4/8/2020 2:52 PM, Jesper Dangaard Brouer wrote:
> The mlx5 driver have multiple memory models, which are also changed
> according to whether a XDP bpf_prog is attached.
> 
> The 'rx_striding_rq' setting is adjusted via ethtool priv-flags e.g.:
>   # ethtool --set-priv-flags mlx5p2 rx_striding_rq off
> 
> On the general case with 4K page_size and regular MTU packet, then
> the frame_sz is 2048 and 4096 when XDP is enabled, in both modes.
> 
> The info on the given frame size is stored differently depending on the
> RQ-mode and encoded in a union in struct mlx5e_rq union wqe/mpwqe.
> In rx striding mode rq->mpwqe.log_stride_sz is either 11 or 12, which
> corresponds to 2048 or 4096 (MLX5_WQ_TYPE_LINKED_LIST_STRIDING_RQ).
> In non-striding mode (MLX5_WQ_TYPE_CYCLIC) the frag_stride is stored
> in rq->wqe.info.arr[0].frag_stride.

Just to clarify, the above description is true as long as we're in the 
Linear SKB memory scheme, this holds when:
1) MTU + headroom + tailroom < PAGE_SIZE, and
2) HW LRO is OFF.

Otherwise, mpwqe.log_stride_sz can be smaller, and frag_stride of 
wqe_info can vary from one index to another.

> 
> To reduce effect on fast-path, this patch determine the frame_sz at
> setup time, to avoid determining the memory model runtime.
> 
> Cc: Tariq Toukan <tariqt@...lanox.com>
> Cc: Saeed Mahameed <saeedm@...lanox.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
>   drivers/net/ethernet/mellanox/mlx5/core/en.h      |    1 +
>   drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c  |    1 +
>   drivers/net/ethernet/mellanox/mlx5/core/en_main.c |    4 ++++
>   3 files changed, 6 insertions(+)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en.h b/drivers/net/ethernet/mellanox/mlx5/core/en.h
> index 12a61bf82c14..1f280fc142ca 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en.h
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en.h
> @@ -651,6 +651,7 @@ struct mlx5e_rq {
>   	struct {
>   		u16            umem_headroom;
>   		u16            headroom;
> +		u32            frame_sz;
>   		u8             map_dir;   /* dma map direction */
>   	} buff;
>   
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> index f049e0ac308a..de4ad2c9f49a 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> @@ -137,6 +137,7 @@ bool mlx5e_xdp_handle(struct mlx5e_rq *rq, struct mlx5e_dma_info *di,
>   	if (xsk)
>   		xdp.handle = di->xsk.handle;
>   	xdp.rxq = &rq->xdp_rxq;
> +	xdp.frame_sz = rq->buff.frame_sz;
>   
>   	act = bpf_prog_run_xdp(prog, &xdp);
>   	if (xsk) {
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> index dd7f338425eb..b9595315c45b 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> @@ -462,6 +462,8 @@ static int mlx5e_alloc_rq(struct mlx5e_channel *c,
>   		rq->mpwqe.num_strides =
>   			BIT(mlx5e_mpwqe_get_log_num_strides(mdev, params, xsk));
>   
> +		rq->buff.frame_sz = (1 << rq->mpwqe.log_stride_sz);
> +

This is always correct.

>   		err = mlx5e_create_rq_umr_mkey(mdev, rq);
>   		if (err)
>   			goto err_rq_wq_destroy;
> @@ -485,6 +487,8 @@ static int mlx5e_alloc_rq(struct mlx5e_channel *c,
>   			num_xsk_frames = wq_sz << rq->wqe.info.log_num_frags;
>   
>   		rq->wqe.info = rqp->frags_info;
> +		rq->buff.frame_sz = rq->wqe.info.arr[0].frag_stride;
> +

This is not always correct.
Size of the last frag for a large MTU might be a full page.
See:
https://elixir.bootlin.com/linux/latest/source/drivers/net/ethernet/mellanox/mlx5/core/en_main.c#L2097

However, you won't try to use this value at all in the non-linear SKB 
flow, as it's not compatible with XDP.

Anyway, I prefer this value to be always true. No matter if it's really 
used or not.
Probably rename the field name to indicate this?
Something like: single_frame_sz / first_frame_sz ?

>   		rq->wqe.frags =
>   			kvzalloc_node(array_size(sizeof(*rq->wqe.frags),
>   					(wq_sz << rq->wqe.info.log_num_frags)),
> 
> 

Thanks,
Tariq

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