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

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Date: Tue, 11 Jul 2023 21:59:44 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Stanislav Fomichev <sdf@...gle.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 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.
I just don't see how xdp tx offloads are moving a needle in that direction.

> I hope we can both agree that with an api like
> mlx5_l4_csum_offload(bool encap) we can't be 100% certain that the
> hw is gonna handle any packet layout? So how is that different
> from a generic api that also can't work in all cases?

If it's hw specific then yes.
Will [mlx5|...]_l4_csum_offload apply to other nics? I doubt.

> AF_XDP is a generic layer for low-level access and it provides generic
> descriptor format, so why suddenly we have this requirement where we have
> to do prog rewrite for every new nic?
>
> Current AF_XDP programs are pretty portable (obviously depend on
> a bunch of nic features), it seems like a good idea to try to preserve
> this property? (again, with an asterisk, where we should allow some
> differentiation, etc, etc)

Agree. AF_XDP needs a generic api that will allow user space
request HW to do TSO, csum offload, etc.
xdp tx and af_xdp are too different to pull under the same framework.
xdp progs will interact with the kernel via kfuncs.
af_xdp needs a different api to express packet geometry and offload requests.
The user space cannot do it with bpf programs.
In case of AF_XDP the bpf prog in the kernel is a filter only.
For the majority of the cases bpf prog is not necessary and shouldn't be
required to express HW offloads.