netdev - Re: [RFC bpf-next 0/7] bpf: netdev TX metadata

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Date: Mon, 12 Jun 2023 23:00:54 +0200
From: Toke Høiland-Jørgensen <toke@...nel.org>
To: Stanislav Fomichev <sdf@...gle.com>, bpf@...r.kernel.org
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 martin.lau@...ux.dev, song@...nel.org, yhs@...com,
 john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
 haoluo@...gle.com, jolsa@...nel.org, willemb@...gle.com,
 dsahern@...nel.org, magnus.karlsson@...el.com, bjorn@...nel.org,
 maciej.fijalkowski@...el.com, netdev@...r.kernel.org
Subject: Re: [RFC bpf-next 0/7] bpf: netdev TX metadata

Some immediate thoughts after glancing through this:

> --- Use cases ---
>
> The goal of this series is to add two new standard-ish places
> in the transmit path:
>
> 1. Right before the packet is transmitted (with access to TX
>    descriptors)
> 2. Right after the packet is actually transmitted and we've received the
>    completion (again, with access to TX completion descriptors)
>
> Accessing TX descriptors unlocks the following use-cases:
>
> - Setting device hints at TX: XDP/AF_XDP might use these new hooks to
> use device offloads. The existing case implements TX timestamp.
> - Observability: global per-netdev hooks can be used for tracing
> the packets and exploring completion descriptors for all sorts of
> device errors.
>
> Accessing TX descriptors also means that the hooks have to be called
> from the drivers.
>
> The hooks are a light-weight alternative to XDP at egress and currently
> don't provide any packet modification abilities. However, eventually,
> can expose new kfuncs to operate on the packet (or, rather, the actual
> descriptors; for performance sake).

dynptr?

> --- UAPI ---
>
> The hooks are implemented in a HID-BPF style. Meaning they don't
> expose any UAPI and are implemented as tracing programs that call
> a bunch of kfuncs. The attach/detach operation happen via BPF syscall
> programs. The series expands device-bound infrastructure to tracing
> programs.

Not a fan of the "attach from BPF syscall program" thing. These are part
of the XDP data path API, and I think we should expose them as proper
bpf_link attachments from userspace with introspection etc. But I guess
the bpf_mprog thing will give us that?

> --- skb vs xdp ---
>
> The hooks operate on a new light-weight devtx_frame which contains:
> - data
> - len
> - sinfo
>
> This should allow us to have a unified (from BPF POW) place at TX
> and not be super-taxing (we need to copy 2 pointers + len to the stack
> for each invocation).

Not sure what I think about this one. At the very least I think we
should expose xdp->data_meta as well. I'm not sure what the use case for
accessing skbs is? If that *is* indeed useful, probably there will also
end up being a use case for accessing the full skb?

> --- Multiprog attachment ---
>
> Currently, attach/detach don't expose links and don't support multiple
> programs. I'm planning to use Daniel's bpf_mprog once it lands.
>
> --- TODO ---
>
> Things that I'm planning to do for the non-RFC series:
> - have some real device support to verify xdp_hw_metadata works

Would be good to see some performance numbers as well :)

> - freplace
> - Documentation/networking/xdp-rx-metadata.rst - like documentation
>
> --- CC ---
>
> CC'ing people only on the cover letter. Hopefully can find the rest via
> lore.

Well, I found it there, even though I was apparently left off the Cc
list :(

-Toke