Re: [bpf-next PATCH] xdp: accept that XDP headroom isn't always equal XDP_PACKET_HEADROOM

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

On Tue, Mar 03, 2020 at 12:46:58PM +0100, Jesper Dangaard Brouer wrote:
> The Intel based drivers (ixgbe + i40e) have implemented XDP with
> headroom 192 bytes and not the recommended 256 bytes defined by
> XDP_PACKET_HEADROOM.  For generic-XDP, accept that this headroom
> is also a valid size.
> 
> Still for generic-XDP if headroom is less, still expand headroom to
> XDP_PACKET_HEADROOM as this is the default in most XDP drivers.
> 
> Tested on ixgbe with xdp_rxq_info --skb-mode and --action XDP_DROP:
> - Before: 4,816,430 pps
> - After : 7,749,678 pps
> (Note that ixgbe in native mode XDP_DROP 14,704,539 pps)
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  include/uapi/linux/bpf.h |    1 +
>  net/core/dev.c           |    4 ++--
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 906e9f2752db..14dc4f9fb3c8 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3312,6 +3312,7 @@ struct bpf_xdp_sock {
>  };
>  
>  #define XDP_PACKET_HEADROOM 256
> +#define XDP_PACKET_HEADROOM_MIN 192

why expose it in uapi?

>  /* User return codes for XDP prog type.
>   * A valid XDP program must return one of these defined values. All other
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 4770dde3448d..9c941cd38b13 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4518,11 +4518,11 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
>  		return XDP_PASS;
>  
>  	/* XDP packets must be linear and must have sufficient headroom
> -	 * of XDP_PACKET_HEADROOM bytes. This is the guarantee that also
> +	 * of XDP_PACKET_HEADROOM_MIN bytes. This is the guarantee that also
>  	 * native XDP provides, thus we need to do it here as well.
>  	 */
>  	if (skb_cloned(skb) || skb_is_nonlinear(skb) ||
> -	    skb_headroom(skb) < XDP_PACKET_HEADROOM) {
> +	    skb_headroom(skb) < XDP_PACKET_HEADROOM_MIN) {
>  		int hroom = XDP_PACKET_HEADROOM - skb_headroom(skb);

this looks odd. It's comparing against 192, but doing math with 256.
I guess that's ok, but needs a clear comment.
How about just doing 'skb_headroom(skb) < 192' here.
Or #define 192 right before this function with a comment about ixgbe?