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

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2023/11/22 14:25, Song Liu wrote:
> On Mon, Nov 20, 2023 at 12:05 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>>
>> On 2023/11/20 6:02, Song Liu wrote:
> [...]
>>>> 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.
>>
>> Documentation/bpf/kfuncs.rst still says:
>>   > kfuncs provide a kernel <-> kernel API, and thus are not bound by any
>>   > of the strict stability restrictions associated with kernel <-> user
>>   > UAPIs.
>>
>> Is it possible to change the statement like as follows:
>> "Most kfuncs provide a kernel <-> kernel API, and thus are not bound by
>> any of the strict stability restrictions associated with kernel <-> user
>> UAPIs. kfuncs that have same stability restrictions associated with
>> UAPIs are exceptional, and must be carefully reviewed by subsystem (and
>> BPF?) maintainers as any other UAPIs are."
> 
> I am afraid this is against the intention to not guarantee UAPI-level stability
> for kfuncs.

Is it possible to ensure that a QEMU binary with the eBPF program 
included works on different kernel versions without UAPI-level stability 
then? Otherwise, I think we need to think of the minimal UAPI addition 
that exposes the feature I propose, and the two options I presented 
first are the candidates of such: the stable BPF change or tuntap 
interface change.

Regards,
Akihiko Odaki