netdev - XDP bpf_tail_call_redirect(): yea or nay?

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Date:   Thu, 7 May 2020 14:20:06 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     bpf <bpf@...r.kernel.org>
Cc:     Netdev <netdev@...r.kernel.org>
Subject: XDP bpf_tail_call_redirect(): yea or nay?

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?

Björn