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

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Date:   Mon, 25 Nov 2019 12:18:19 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Björn Töpel <bjorn.topel@...il.com>,
        netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        bpf@...r.kernel.org, magnus.karlsson@...il.com,
        magnus.karlsson@...el.com, jonathan.lemon@...il.com,
        ecree@...arflare.com, andrii.nakryiko@...il.com,
        tariqt@...lanox.com, saeedm@...lanox.com, maximmi@...lanox.com
Subject: Re: [PATCH bpf-next v2 2/6] xdp: introduce xdp_call

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...

-Toke