netdev - Re: [PATCH bpf v2 2/3] bpf_fib_lookup: optionally skip neighbour lookup

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Date:   Tue, 20 Oct 2020 07:49:01 -0600
From:   David Ahern <dsahern@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf v2 2/3] bpf_fib_lookup: optionally skip neighbour
 lookup

On 10/20/20 4:51 AM, Toke Høiland-Jørgensen wrote:
> From: Toke Høiland-Jørgensen <toke@...hat.com>
> 
> The bpf_fib_lookup() helper performs a neighbour lookup for the destination
> IP and returns BPF_FIB_LKUP_NO_NEIGH if this fails, with the expectation
> that the BPF program will deal with this condition, either by passing the
> packet up the stack, or by using bpf_redirect_neigh().
> 
> The neighbour lookup is done via a hash table (through ___neigh_lookup_noref()),
> which incurs some overhead. If the caller knows this is likely to fail
> anyway, it may want to skip that and go unconditionally to
> bpf_redirect_neigh(). For this use case, add a flag to bpf_fib_lookup()
> that will make it skip the neighbour lookup and instead always return
> BPF_FIB_LKUP_RET_NO_NEIGH (but still populate the gateway and target
> ifindex).
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>  include/uapi/linux/bpf.h       |   10 ++++++----
>  net/core/filter.c              |   16 ++++++++++++++--
>  tools/include/uapi/linux/bpf.h |   10 ++++++----
>  3 files changed, 26 insertions(+), 10 deletions(-)

Nack. Please don't.

As I mentioned in my reply to Daniel, I would prefer such logic be
pushed to the bpf programs. There is no reason for rare run time events
to warrant a new flag and new check in the existing FIB helpers. The bpf
programs can take the hit of the extra lookup.