netdev - Re: [RFC PATCH bpf-next 2/2] i40e: avoid xdp_do_redirect() call when "redirect_tail

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Date:   Tue, 09 Jun 2020 21:47:46 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Björn Töpel <bjorn.topel@...il.com>,
        ast@...nel.org, daniel@...earbox.net, bpf@...r.kernel.org,
        john.fastabend@...il.com
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        magnus.karlsson@...el.com, netdev@...r.kernel.org,
        brouer@...hat.com, maciej.fijalkowski@...el.com
Subject: Re: [RFC PATCH bpf-next 2/2] i40e: avoid xdp_do_redirect() call when "redirect_tail_call" is set

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

> From: Björn Töpel <bjorn.topel@...el.com>
>
> If an XDP program, where all the bpf_redirect_map() calls are tail
> calls (as defined by the previous commit), the driver does not need to
> explicitly call xdp_do_redirect().
>
> The driver checks the active XDP program, and notifies the BPF helper
> indirectly via xdp_set_redirect_tailcall().
>
> This is just a naive, as-simple-as-possible implementation, calling
> xdp_set_redirect_tailcall() for each packet.

Do you really need the driver changes? The initial setup could be moved
to bpf_prog_run_xdp(), and xdp_do_redirect() could be changed to an
inline wrapper that just checks a flag and immediately returns 0 if the
redirect action was already performed. Or am I missing some reason why
this wouldn't work?

-Toke