Re: [RFC PATCH v2 1/7] bpf: Introduce BPF_PROG_TYPE_VNET_HASH

[Song Liu]

On Sun, Nov 19, 2023 at 12:03 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
>
[...]
>
> Unfortunately no. The communication with the userspace can be done with
> two different means:
> - usual socket read/write
> - vhost for direct interaction with a KVM guest
>
> The BPF map may be a valid option for socket read/write, but it is not
> for vhost. In-kernel vhost may fetch hash from the BPF map, but I guess
> it's not a standard way to have an interaction between the kernel code
> and a BPF program.

I am very new to areas like vhost and KVM. So I don't really follow.
Does this mean we have the guest kernel reading data from host eBPF
programs (loaded by Qemu)?

> >
> >>
> >> Unfortunately, however, it is not acceptable for the BPF subsystem
> >> because the "stable" BPF is completely fixed these days. The
> >> "unstable/kfunc" BPF is an alternative, but the eBPF program will be
> >> shipped with a portable userspace program (QEMU)[1] so the lack of
> >> interface stability is not tolerable.
> >
> > bpf kfuncs are as stable as exported symbols. Is exported symbols
> > like stability enough for the use case? (I would assume yes.)
> >
> >>
> >> Another option is to hardcode the algorithm that was conventionally
> >> implemented with eBPF steering program in the kernel[2]. It is possible
> >> because the algorithm strictly follows the virtio-net specification[3].
> >> However, there are proposals to add different algorithms to the
> >> specification[4], and hardcoding the algorithm to the kernel will
> >> require to add more UAPIs and code each time such a specification change
> >> happens, which is not good for tuntap.
> >
> > The requirement looks similar to hid-bpf. Could you explain why that
> > model is not enough? HID also requires some stability AFAICT.
>
> I have little knowledge with hid-bpf, but I assume it is more like a
> "safe" kernel module; in my understanding, it affects the system state
> and is intended to be loaded with some kind of a system daemon. It is
> fine to have the same lifecycle with the kernel for such a BPF program;
> whenever the kernel is updated, the distributor can recompile the BPF
> program with the new kernel headers and ship it along with the kernel
> just as like a kernel module.
>
> In contrast, our intended use case is more like a normal application.
> So, for example, a user may download a container and run QEMU (including
> the BPF program) installed in the container. As such, it is nice if the
> ABI is stable across kernel releases, but it is not guaranteed for
> kfuncs. Such a use case is already covered with the eBPF steering
> program so I want to maintain it if possible.

TBH, I don't think stability should be a concern for kfuncs used by QEMU.
Many core BPF APIs are now implemented as kfuncs: bpf_dynptr_*,
bpf_rcu_*, etc. As long as there are valid use cases,these kfuncs will
be supported.

Thanks,
Song