linux-kernel - Re: Should I add BPF kfuncs for userspace apps? And how?

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Date: Fri, 15 Dec 2023 08:36:44 -0800
From: Stephen Hemminger <stephen@...workplumber.org>
To: Akihiko Odaki <akihiko.odaki@...nix.com>
Cc: Benjamin Tissoires <benjamin.tissoires@...hat.com>, Alexei Starovoitov
 <alexei.starovoitov@...il.com>, Jason Wang <jasowang@...hat.com>, Alexei
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 <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
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 <shuah@...nel.org>, Yuri Benditovich <yuri.benditovich@...nix.com>, Andrew
 Melnychenko <andrew@...nix.com>, Benjamin Tissoires <bentiss@...nel.org>,
 bpf <bpf@...r.kernel.org>, "open list:DOCUMENTATION"
 <linux-doc@...r.kernel.org>, kvm@...r.kernel.org, LKML
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 "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>,
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Subject: Re: Should I add BPF kfuncs for userspace apps? And how?

On Fri, 15 Dec 2023 14:49:56 +0900
Akihiko Odaki <akihiko.odaki@...nix.com> wrote:

> >> It is exactly what BPF_PROG_TYPE_SOCKET_FILTER does, but it lacks a
> >> mechanism to report hash values so I need to extend it or invent a new
> >> method. Extending BPF_PROG_TYPE_SOCKET_FILTER is not a way forward since
> >> CO-RE is superior to the context rewrite it relies on. But apparently
> >> adopting kfuncs and CO-RE also means to lose the "we don't break user
> >> space" contract although I have no intention to expose kernel internals
> >> to the eBPF program.  
> > 
> > An example is how one part of DPDK recomputes RSS over TAP.
> > 
> > https://git.dpdk.org/dpdk/tree/drivers/net/tap/bpf/tap_bpf_program.c
> > 
> > This feature is likely to be removed, because it is not actively used
> > and the changes in BPF program loading broke it on current kernel
> > releases.  Which brings up the point that since the kernel does
> > not have stable API/ABI for BPF program infrastructure, I would
> > avoid it for projects that don't want to deal with that.  
> 
> It's unfortunate to hear that, but thanks for the information.
> I'll consider more about the option not using BPF (plain ioctl and 
> in-kernel implementation).

With libbpf, things are much better. It is just that projects like
DPDK have to support wide range of kernels including older versions of RHEL.