netdev - Re: [RFC PATCH bpf-next] bpf, xdp: per-map bpf_redirect

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Date:   Fri, 29 Jan 2021 17:45:27 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Björn Töpel <bjorn.topel@...il.com>,
        ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        magnus.karlsson@...el.com, maciej.fijalkowski@...el.com,
        kuba@...nel.org, jonathan.lemon@...il.com, maximmi@...dia.com,
        davem@...emloft.net, hawk@...nel.org, john.fastabend@...il.com
Subject: Re: [RFC PATCH bpf-next] bpf, xdp: per-map bpf_redirect_map
 functions for XDP

Björn Töpel <bjorn.topel@...il.com> writes:

> From: Björn Töpel <bjorn.topel@...el.com>
>
> Currently the bpf_redirect_map() implementation dispatches to the
> correct map-lookup function via a switch-statement. To avoid the
> dispatching, this change adds one bpf_redirect_map() implementation per
> map. Correct function is automatically selected by the BPF verifier.
>
> Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
> ---
> Hi XDP-folks!
>
> This is another take on my bpf_redirect_xsk() patch [1]. I figured I
> send it as an RFC for some early input. My plan is to include it as
> part of the xdp_do_redirect() optimization of [1].

Assuming the maintainers are OK with the special-casing in the verifier,
this looks like a neat way to avoid the runtime overhead to me. The
macro hackery is not the prettiest; I wonder if the same effect could be
achieved by using inline functions? If not, at least a comment
explaining the reasoning (and that the verifier will substitute the
right function) might be nice? Mostly in relation to this bit:

>  static const struct bpf_func_proto bpf_xdp_redirect_map_proto = {
> -	.func           = bpf_xdp_redirect_map,
> +	.func           = bpf_xdp_redirect_devmap,

Ah, if only we were writing the kernel in a language with proper macro
support... One can dream! :)

>> For AF_XDP rxdrop this yields +600Mpps. I'll do CPU/DEVMAP
>> measurements for the patch proper.
>>
>
> Kpps, not Mpps. :-P

Aww, too bad ;)
Still, nice!

-Toke