netdev - Re: [RFC bpf-next v3 09/14] net/mlx5e: Implement devtx kfuncs

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Message-ID: <20230711200740.236b0142@kernel.org>
Date: Tue, 11 Jul 2023 20:07:40 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Stanislav Fomichev <sdf@...gle.com>, bpf <bpf@...r.kernel.org>, Alexei
 Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, Yonghong Song
 <yhs@...com>, John Fastabend <john.fastabend@...il.com>, KP Singh
 <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>, Jiri Olsa
 <jolsa@...nel.org>, Toke Høiland-Jørgensen
 <toke@...nel.org>, Willem de Bruijn <willemb@...gle.com>, David Ahern
 <dsahern@...nel.org>, "Karlsson, Magnus" <magnus.karlsson@...el.com>,
 Björn Töpel <bjorn@...nel.org>, "Fijalkowski, Maciej"
 <maciej.fijalkowski@...el.com>, Jesper Dangaard Brouer <hawk@...nel.org>,
 Network Development <netdev@...r.kernel.org>, xdp-hints@...-project.net
Subject: Re: [RFC bpf-next v3 09/14] net/mlx5e: Implement devtx kfuncs

On Tue, 11 Jul 2023 19:37:23 -0700 Alexei Starovoitov wrote:
> > I hope I'm not misremembering but I think I suggested at the beginning
> > to create a structure describing packet geometry and requested offloads,
> > and for the prog fill that in.  
> 
> hmm. but that's what skb is for. skb == packet geometry ==
> layout of headers, payload, inner vs outer, csum partial, gso params.
> 
> bpf at tc layer supposed to interact with that correctly.
> If the packet is modified skb geometry should be adjusted accordingly.
> Like BPF_F_RECOMPUTE_CSUM flag in bpf_skb_store_bytes().
> 
> > All operating systems I know end up doing that, we'll end up doing
> > that as well. The question is whether we're willing to learn from
> > experience or prefer to go on a wild ride first...  
> 
> I don't follow. This thread was aimed to add xdp layer knobs.
> To me XDP is a driver level.

Driver is not a layer of the networking stack, I don't think it's 
a useful or meaningful anchor point for the mental model. 

We're talking about a set of functionality, we can look at how that
functionality was exposed in existing code.

> 'struct xdp_md' along with
> BPF_F_XDP_HAS_FRAGS is the best abstraction we could do generalizing
> dma-buffers (page and multi-page) that drivers operate on.

I think you're working against your own claim.
xdp frags explicitly reuse struct skb_shared_info.

> Everything else at driver level is too unique to generalize.
> skb layer is already doing its job.

How can you say that drivers are impossible to generalize and than
that the skb layer "is doing its job" ie. generalizes them?

> In that sense "generic XDP" is a feature for testing only.
> Trying to make "generic XDP" fast is missing the point of XDP.

That's a topic on its own.

> AF_XDP is a different concept. Exposing timestamp,
> csum, TSO to AF_XDP users is a different design challenge.
> I'm all for doing that, but trying to combine "timestamp in xdp tx"
> and "timestamp in AF_XDP" will lead to bad trade-off-s for both.
> Which I think this patchset demonstrates.

Too vague to agree or disagree with.