netdev - Re: [PATCH bpf-next v2] libbpf: use implicit XSKMAP lookup from AF

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Date:   Mon, 21 Oct 2019 15:37:54 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        bpf <bpf@...r.kernel.org>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Subject: Re: [PATCH bpf-next v2] libbpf: use implicit XSKMAP lookup from
 AF_XDP XDP program

On Mon, 21 Oct 2019 at 14:19, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Björn Töpel <bjorn.topel@...il.com> writes:
>
[...]
> >
> > bpf_redirect_map() returns a 32-bit signed int, so the upper 32-bit
> > will need to be cleared. Having an explicit AND is one instruction
> > less than two shifts. So, it's an optimization (every instruction is
> > sacred).
>
> OIC. Well, a comment explaining that might be nice (since you're doing
> per-instruction comments anyway)? :)
>

Sure, I can do a v3 with a comment, unless someone has a better idea
avoiding both shifts and AND.

Thanks for taking a look!


Björn


> -Toke
>