Subject: Re: [PATCHv9 bpf-next 2/4] xdp: extend xdp_redirect_map with broadcast support

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 22 Apr 2021 18:53:32 +0200

On Thu, 22 Apr 2021 15:14:52 +0800
Hangbin Liu <[email-masked]> wrote:

> diff --git a/net/core/filter.c b/net/core/filter.c
> index cae56d08a670..afec192c3b21 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
[...]
>  int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
>  		    struct bpf_prog *xdp_prog)
>  {
> @@ -3933,6 +3950,7 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
>  	enum bpf_map_type map_type = ri->map_type;
>  	void *fwd = ri->tgt_value;
>  	u32 map_id = ri->map_id;
> +	struct bpf_map *map;
>  	int err;
>  
>  	ri->map_id = 0; /* Valid map id idr range: [1,INT_MAX[ */
> @@ -3942,7 +3960,12 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
>  	case BPF_MAP_TYPE_DEVMAP:
>  		fallthrough;
>  	case BPF_MAP_TYPE_DEVMAP_HASH:
> -		err = dev_map_enqueue(fwd, xdp, dev);
> +		map = xchg(&ri->map, NULL);

Hmm, this looks dangerous for performance to have on this fast-path.
The xchg call can be expensive, AFAIK this is an atomic operation.

> +		if (map)
> +			err = dev_map_enqueue_multi(xdp, dev, map,
> +						    ri->flags & BPF_F_EXCLUDE_INGRESS);
> +		else
> +			err = dev_map_enqueue(fwd, xdp, dev);
>  		break;
>  	case BPF_MAP_TYPE_CPUMAP:
>  		err = cpu_map_enqueue(fwd, xdp, dev);
> @@ -3984,13 +4007,19 @@ static int xdp_do_generic_redirect_map(struct net_device *dev,
>  				       enum bpf_map_type map_type, u32 map_id)
>  {
>  	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
> +	struct bpf_map *map;
>  	int err;
>  
>  	switch (map_type) {
>  	case BPF_MAP_TYPE_DEVMAP:
>  		fallthrough;
>  	case BPF_MAP_TYPE_DEVMAP_HASH:
> -		err = dev_map_generic_redirect(fwd, skb, xdp_prog);
> +		map = xchg(&ri->map, NULL);

Same here!

> +		if (map)
> +			err = dev_map_redirect_multi(dev, skb, xdp_prog, map,
> +						     ri->flags & BPF_F_EXCLUDE_INGRESS);
> +		else
> +			err = dev_map_generic_redirect(fwd, skb, xdp_prog);
>  		if (unlikely(err))
>  			goto err;
>  		break;

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer