netdev - Re: XDP bpf_tail_call_redirect(): yea or nay?

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Date:   Thu, 07 May 2020 15:44:16 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Björn Töpel <bjorn.topel@...il.com>,
        bpf <bpf@...r.kernel.org>
Cc:     Netdev <netdev@...r.kernel.org>
Subject: Re: XDP bpf_tail_call_redirect(): yea or nay?

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

> Before I start hacking on this, I might as well check with the XDP
> folks if this considered a crappy idea or not. :-)
>
> The XDP redirect flow for a packet is typical a dance of
> bpf_redirect_map() that updates the bpf_redirect_info structure with
> maps type/items, which is then followed by an xdp_do_redirect(). That
> function takes an action based on the bpf_redirect_info content.
>
> I'd like to get rid of the xdp_do_redirect() call, and the
> bpf_redirect_info (per-cpu) lookup. The idea is to introduce a new
> (oh-no!) XDP action, say, XDP_CONSUMED and a built-in helper with
> tail-call semantics.
>
> Something across the lines of:
>
> --8<--
>
> struct {
>         __uint(type, BPF_MAP_TYPE_XSKMAP);
>         __uint(max_entries, MAX_SOCKS);
>         __uint(key_size, sizeof(int));
>         __uint(value_size, sizeof(int));
> } xsks_map SEC(".maps");
>
> SEC("xdp1")
> int xdp_prog1(struct xdp_md *ctx)
> {
>         bpf_tail_call_redirect(ctx, &xsks_map, 0);
>         // Redirect the packet to an AF_XDP socket at entry 0 of the
>         // map.
>         //
>         // After a successful call, ctx is said to be
>         // consumed. XDP_CONSUMED will be returned by the program.
>         // Note that if the call is not successful, the buffer is
>         // still valid.
>         //
>         // XDP_CONSUMED in the driver means that the driver should not
>         // issue an xdp_do_direct() call, but only xdp_flush().
>         //
>         // The verifier need to be taught that XDP_CONSUMED can only
>         // be returned "indirectly", meaning a bpf_tail_call_XXX()
>         // call. An explicit "return XDP_CONSUMED" should be
>         // rejected. Can that be implemented?
>         return XDP_PASS; // or any other valid action.
> }
>
> -->8--
>
> The bpf_tail_call_redirect() would work with all redirectable maps.
>
> Thoughts? Tomatoes? Pitchforks?

The above answers the 'what'. Might be easier to evaluate if you also
included the 'why'? :)

-Toke