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

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Date: Tue, 11 Jul 2023 22:36:08 -0700
From: Stanislav Fomichev <sdf@...gle.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: 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>, Jakub Kicinski <kuba@...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, Jul 11, 2023 at 9:59 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Jul 11, 2023 at 8:29 PM Stanislav Fomichev <sdf@...gle.com> wrote:
> >
> >
> > This will slow things down, but not to the point where it's on par
> > with doing sw checksum. At least in theory.
> > We can't stay at skb when using AF_XDP. AF_XDP would benefit from having
> > the offloads.
>
> To clarify: yes, AF_XDP needs generalized HW offloads.

Great! To reiterate, I'm mostly interested in af_xdp wrt tx
timestamps. So if the consensus is not to mix xdp-tx and af_xdp-tx,
I'm fine with switching to adding some fixed af_xdp descriptor format
to enable offloads on tx.

> I just don't see how xdp tx offloads are moving a needle in that direction.

Let me try to explain how both might be similar, maybe I wasn't clear
enough on that.
For af_xdp tx packet, the userspace puts something in the af_xdp frame
metadata area (headrom) which then gets executed/interpreted by the
bpf program at devtx (which calls kfuncs to enable particular
offloads).
IOW, instead of defining some fixed layout for the tx offloads, the
userspace and bpf program have some agreement on the layout (and bpf
program "applies" the offloads by calling the kfuncs).
Also (in theory) the same hooks can be used for xdp-tx.
Does it make sense? But, again, happy to scratch that whole idea if
we're fine with a fixed layout for af_xdp.