netdev - Re: [RFC PATCH v2 1/7] bpf: Introduce BPF_PROG_TYPE_VNET

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Date: Tue, 17 Oct 2023 12:03:48 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Jason Wang <jasowang@...hat.com>
Cc: Akihiko Odaki <akihiko.odaki@...nix.com>, 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 <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Jonathan Corbet <corbet@....net>, 
	Willem de Bruijn <willemdebruijn.kernel@...il.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, bpf <bpf@...r.kernel.org>, 
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Network Development <netdev@...r.kernel.org>, kvm@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, 
	Yuri Benditovich <yuri.benditovich@...nix.com>, Andrew Melnychenko <andrew@...nix.com>
Subject: Re: [RFC PATCH v2 1/7] bpf: Introduce BPF_PROG_TYPE_VNET_HASH

On Mon, Oct 16, 2023 at 7:38 PM Jason Wang <jasowang@...hat.com> wrote:
>
> On Tue, Oct 17, 2023 at 7:53 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Sun, Oct 15, 2023 at 10:10 AM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
> > >
> > > On 2023/10/16 1:07, Alexei Starovoitov wrote:
> > > > On Sun, Oct 15, 2023 at 7:17 AM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
> > > >>
> > > >> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > >> index 0448700890f7..298634556fab 100644
> > > >> --- a/include/uapi/linux/bpf.h
> > > >> +++ b/include/uapi/linux/bpf.h
> > > >> @@ -988,6 +988,7 @@ enum bpf_prog_type {
> > > >>          BPF_PROG_TYPE_SK_LOOKUP,
> > > >>          BPF_PROG_TYPE_SYSCALL, /* a program that can execute syscalls */
> > > >>          BPF_PROG_TYPE_NETFILTER,
> > > >> +       BPF_PROG_TYPE_VNET_HASH,
> > > >
> > > > Sorry, we do not add new stable program types anymore.
> > > >
> > > >> @@ -6111,6 +6112,10 @@ struct __sk_buff {
> > > >>          __u8  tstamp_type;
> > > >>          __u32 :24;              /* Padding, future use. */
> > > >>          __u64 hwtstamp;
> > > >> +
> > > >> +       __u32 vnet_hash_value;
> > > >> +       __u16 vnet_hash_report;
> > > >> +       __u16 vnet_rss_queue;
> > > >>   };
> > > >
> > > > we also do not add anything to uapi __sk_buff.
> > > >
> > > >> +const struct bpf_verifier_ops vnet_hash_verifier_ops = {
> > > >> +       .get_func_proto         = sk_filter_func_proto,
> > > >> +       .is_valid_access        = sk_filter_is_valid_access,
> > > >> +       .convert_ctx_access     = bpf_convert_ctx_access,
> > > >> +       .gen_ld_abs             = bpf_gen_ld_abs,
> > > >> +};
> > > >
> > > > and we don't do ctx rewrites like this either.
> > > >
> > > > Please see how hid-bpf and cgroup rstat are hooking up bpf
> > > > in _unstable_ way.
> > >
> > > Can you describe what "stable" and "unstable" mean here? I'm new to BPF
> > > and I'm worried if it may mean the interface stability.
> > >
> > > Let me describe the context. QEMU bundles an eBPF program that is used
> > > for the "eBPF steering program" feature of tun. Now I'm proposing to
> > > extend the feature to allow to return some values to the userspace and
> > > vhost_net. As such, the extension needs to be done in a way that ensures
> > > interface stability.
> >
> > bpf is not an option then.
> > we do not add stable bpf program types or hooks any more.
>
> Does this mean eBPF could not be used for any new use cases other than
> the existing ones?

It means that any new use of bpf has to be unstable for the time being.

> > If a kernel subsystem wants to use bpf it needs to accept the fact
> > that such bpf extensibility will be unstable and subsystem maintainers
> > can decide to remove such bpf support in the future.
>
> I don't see how it is different from the existing ones.

Can we remove BPF_CGROUP_RUN_PROG_INET_INGRESS hook along
with BPF_PROG_TYPE_CGROUP_SKB program type?
Obviously not.
We can refactor it. We can move it around, but not remove.
That's the difference in stable vs unstable.