Re: [PATCH v10 05/12] net/mlx4_en: add support for fast rx drop bpf program

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Jul 19, 2016 at 12:16:50PM -0700, Brenden Blanco wrote:
> Add support for the BPF_PROG_TYPE_XDP hook in mlx4 driver.
> 
> In tc/socket bpf programs, helpers linearize skb fragments as needed
> when the program touches the packet data. However, in the pursuit of
> speed, XDP programs will not be allowed to use these slower functions,
> especially if it involves allocating an skb.
> 
> Therefore, disallow MTU settings that would produce a multi-fragment
> packet that XDP programs would fail to access. Future enhancements could
> be done to increase the allowable MTU.
> 
> The xdp program is present as a per-ring data structure, but as of yet
> it is not possible to set at that granularity through any ndo.
> 
> Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>
...
> +static int mlx4_xdp_set(struct net_device *dev, struct bpf_prog *prog)
> +{
> +	struct mlx4_en_priv *priv = netdev_priv(dev);
> +	struct bpf_prog *old_prog;
> +	int xdp_ring_num;
> +	int i;
> +
> +	xdp_ring_num = prog ? ALIGN(priv->rx_ring_num, MLX4_EN_NUM_UP) : 0;
> +
> +	if (priv->num_frags > 1) {
> +		en_err(priv, "Cannot set XDP if MTU requires multiple frags\n");
> +		return -EOPNOTSUPP;
> +	}
> +
> +	if (prog) {
> +		prog = bpf_prog_add(prog, priv->rx_ring_num - 1);
> +		if (IS_ERR(prog))
> +			return PTR_ERR(prog);
> +	}
> +
> +	priv->xdp_ring_num = xdp_ring_num;
> +
> +	/* This xchg is paired with READ_ONCE in the fast path */
> +	for (i = 0; i < priv->rx_ring_num; i++) {
> +		old_prog = xchg(&priv->rx_ring[i]->xdp_prog, prog);
> +		if (old_prog)
> +			bpf_prog_put(old_prog);
> +	}

priv->xdp_ring_num looks similar priv->rx_ring_num, so on the first glance
it seemed that the per ring refactoring broke detach logic, but no. it's good.
Acked-by: Alexei Starovoitov <ast@kernel.org>