netdev - Re: [PATCH bpf-next v2 2/6] xdp: introduce xdp

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Date:   Mon, 25 Nov 2019 16:56:52 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Björn Töpel <bjorn.topel@...il.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>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Edward Cree <ecree@...arflare.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Maxim Mikityanskiy <maximmi@...lanox.com>
Subject: Re: [PATCH bpf-next v2 2/6] xdp: introduce xdp_call

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

> On Mon, 25 Nov 2019 at 12:18, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>
>> Björn Töpel <bjorn.topel@...il.com> writes:
>>
>> > From: Björn Töpel <bjorn.topel@...el.com>
>> >
>> > The xdp_call.h header wraps a more user-friendly API around the BPF
>> > dispatcher. A user adds a trampoline/XDP caller using the
>> > DEFINE_XDP_CALL macro, and updates the BPF dispatcher via
>> > xdp_call_update(). The actual dispatch is done via xdp_call().
>> >
>> > Note that xdp_call() is only supported for builtin drivers. Module
>> > builds will fallback to bpf_prog_run_xdp().
>>
>> I don't like this restriction. Distro kernels are not likely to start
>> shipping all the network drivers builtin, so they won't benefit from the
>> performance benefits from this dispatcher.
>>
>> What is the reason these dispatcher blocks have to reside in the driver?
>> Couldn't we just allocate one system-wide, and then simply change
>> bpf_prog_run_xdp() to make use of it transparently (from the driver
>> PoV)? That would also remove the need to modify every driver...
>>
>
> Good idea! I'll try that out. Thanks for the suggestion!

Awesome! I guess the table may need to be a bit bigger if it's
system-wide? But since you've already gone to all that trouble with the
binary search, I guess that shouldn't have too much of a performance
impact? Maybe the size could even be a config option so users/distros
can make their own size tradeoff?

-Toke