Subject: Re: [PATCH bpf-next v2] libbpf: use implicit XSKMAP lookup from AF_XDP XDP program

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 21 Oct 2019 13:50:25 +0200

Björn Töpel <[email-masked]> writes:

> From: Björn Töpel <[email-masked]>
>
> In commit 43e74c0267a3 ("bpf_xdp_redirect_map: Perform map lookup in
> eBPF helper") the bpf_redirect_map() helper learned to do map lookup,
> which means that the explicit lookup in the XDP program for AF_XDP is
> not needed for post-5.3 kernels.
>
> This commit adds the implicit map lookup with default action, which
> improves the performance for the "rx_drop" [1] scenario with ~4%.
>
> For pre-5.3 kernels, the bpf_redirect_map() returns XDP_ABORTED, and a
> fallback path for backward compatibility is entered, where explicit
> lookup is still performed. This means a slight regression for older
> kernels (an additional bpf_redirect_map() call), but I consider that a
> fair punishment for users not upgrading their kernels. ;-)
>
> v1->v2: Backward compatibility (Toke) [2]
>
> [1] # xdpsock -i eth0 -z -r
> [2] https://lore.kernel.org/bpf/[email-masked]/
>
> Suggested-by: Toke Høiland-Jørgensen <[email-masked]>
> Signed-off-by: Björn Töpel <[email-masked]>
> ---
>  tools/lib/bpf/xsk.c | 45 +++++++++++++++++++++++++++++++++++----------
>  1 file changed, 35 insertions(+), 10 deletions(-)
>
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index b0f532544c91..391a126b3fd8 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -274,33 +274,58 @@ static int xsk_load_xdp_prog(struct xsk_socket *xsk)
>  	/* This is the C-program:
>  	 * SEC("xdp_sock") int xdp_sock_prog(struct xdp_md *ctx)
>  	 * {
> -	 *     int index = ctx->rx_queue_index;
> +	 *     int ret, index = ctx->rx_queue_index;
>  	 *
>  	 *     // A set entry here means that the correspnding queue_id
>  	 *     // has an active AF_XDP socket bound to it.
> +	 *     ret = bpf_redirect_map(&xsks_map, index, XDP_PASS);
> +	 *     ret &= XDP_PASS | XDP_REDIRECT;

Why the masking? Looks a bit weird (XDP return codes are not defined as
bitmask values), and it's not really needed, is it?

-Toke