netdev - Re: [PATCH bpf-next v3 06/11] mlx5e: modify driver for handling offsets

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Date:   Thu, 25 Jul 2019 10:15:55 +0000
From:   Maxim Mikityanskiy <maximmi@...lanox.com>
To:     Kevin Laatz <kevin.laatz@...el.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "bjorn.topel@...el.com" <bjorn.topel@...el.com>,
        "magnus.karlsson@...el.com" <magnus.karlsson@...el.com>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "jonathan.lemon@...il.com" <jonathan.lemon@...il.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        "stephen@...workplumber.org" <stephen@...workplumber.org>,
        "bruce.richardson@...el.com" <bruce.richardson@...el.com>,
        "ciara.loftus@...el.com" <ciara.loftus@...el.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>
Subject: Re: [PATCH bpf-next v3 06/11] mlx5e: modify driver for handling
 offsets

On 2019-07-24 08:10, Kevin Laatz wrote:
> With the addition of the unaligned chunks option, we need to make sure we
> handle the offsets accordingly based on the mode we are currently running
> in. This patch modifies the driver to appropriately mask the address for
> each case.
> 
> Signed-off-by: Kevin Laatz <kevin.laatz@...el.com>
> 
> ---
> v3:
>    - Use new helper function to handle offset
> ---
>   drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c    | 8 ++++++--
>   drivers/net/ethernet/mellanox/mlx5/core/en/xsk/tx.c | 9 +++++++--
>   2 files changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> index b0b982cf69bb..d5245893d2c8 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> @@ -122,6 +122,7 @@ bool mlx5e_xdp_handle(struct mlx5e_rq *rq, struct mlx5e_dma_info *di,
>   		      void *va, u16 *rx_headroom, u32 *len, bool xsk)
>   {
>   	struct bpf_prog *prog = READ_ONCE(rq->xdp_prog);
> +	struct xdp_umem *umem = rq->umem;
>   	struct xdp_buff xdp;
>   	u32 act;
>   	int err;
> @@ -138,8 +139,11 @@ bool mlx5e_xdp_handle(struct mlx5e_rq *rq, struct mlx5e_dma_info *di,
>   	xdp.rxq = &rq->xdp_rxq;
>   
>   	act = bpf_prog_run_xdp(prog, &xdp);
> -	if (xsk)
> -		xdp.handle += xdp.data - xdp.data_hard_start;
> +	if (xsk) {
> +		u64 off = xdp.data - xdp.data_hard_start;
> +
> +		xdp.handle = xsk_umem_handle_offset(umem, xdp.handle, off);
> +	}

What's missed is that umem_headroom is added to handle directly in 
mlx5e_xsk_page_alloc_umem. In my understanding umem_headroom should go 
to the offset part (high 16 bits) of the handle, to 
xsk_umem_handle_offset has to support increasing the offset.

>   	switch (act) {
>   	case XDP_PASS:
>   		*rx_headroom = xdp.data - xdp.data_hard_start;
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/tx.c b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/tx.c
> index 35e188cf4ea4..f596e63cba00 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/tx.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/tx.c
> @@ -61,6 +61,7 @@ bool mlx5e_xsk_tx(struct mlx5e_xdpsq *sq, unsigned int budget)
>   	struct mlx5e_xdp_xmit_data xdptxd;
>   	bool work_done = true;
>   	bool flush = false;
> +	u64 addr, offset;
>   
>   	xdpi.mode = MLX5E_XDP_XMIT_MODE_XSK;
>   
> @@ -82,8 +83,12 @@ bool mlx5e_xsk_tx(struct mlx5e_xdpsq *sq, unsigned int budget)
>   			break;
>   		}
>   
> -		xdptxd.dma_addr = xdp_umem_get_dma(umem, desc.addr);
> -		xdptxd.data = xdp_umem_get_data(umem, desc.addr);
> +		/* for unaligned chunks need to take offset from upper bits */
> +		offset = (desc.addr >> XSK_UNALIGNED_BUF_OFFSET_SHIFT);
> +		addr = (desc.addr & XSK_UNALIGNED_BUF_ADDR_MASK);
> +
> +		xdptxd.dma_addr = xdp_umem_get_dma(umem, addr + offset);
> +		xdptxd.data = xdp_umem_get_data(umem, addr + offset);

Why can't these calculations be encapsulated into 
xdp_umem_get_{dma,data}? I think they are common for all drivers, aren't 
they?

Even if there is some reason not to put this bitshifting stuff into 
xdp_umem_get_* functions, I suggest to encapsulate it into a function 
anyway, because it's a good idea to keep those calculations in a single 
place.

>   		xdptxd.len = desc.len;
>   
>   		dma_sync_single_for_device(sq->pdev, xdptxd.dma_addr,
>