netdev - Re: [PATCH bpf-next V1 2/6] bpf: bpf_fib_lookup return MTU value as output when looked up

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Message-ID: <23e087e7-066c-2228-8df7-3a6b81ad2ba0@gmail.com>
Date:   Wed, 7 Oct 2020 09:38:56 -0700
From:   David Ahern <dsahern@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Maciej Żenczykowski <maze@...gle.com>
Cc:     bpf <bpf@...r.kernel.org>, Linux NetDev <netdev@...r.kernel.org>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Shaun Crampton <shaun@...era.io>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Marek Majkowski <marek@...udflare.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH bpf-next V1 2/6] bpf: bpf_fib_lookup return MTU value as
 output when looked up

On 10/7/20 12:42 AM, Jesper Dangaard Brouer wrote:
> 
> The struct bpf_fib_lookup is exactly 1 cache-line (64 bytes) for
> performance reasons.  I do believe that it can be extended, as Ahern
> designed the BPF-helper API cleverly via a plen (detail below signature).

Yes, I kept it to 64B for performance reasons which is why most fields
have 1 value on input and another on output.

Technically it can be extended, but any cost in doing so should be
abosrbed by the new feature(s). Meaning, users just doing a fib lookup
based on current API should not take a hit with the extra size.