Cong Wang <xiyou.wangcong@xxxxxxxxx> writes: > On Fri, Feb 03, 2023 at 09:57:27AM -0600, David Vernet wrote: >> BPF kernel <-> kernel API stability has been discussed at length over >> the last several weeks and months. Now that we've largely aligned over >> kfuncs being the way forward, and BPF helpers being considered >> functionally frozen, it's time to document the expectations for kfunc >> lifecycles and stability so that everyone (BPF users, kfunc developers, >> and maintainers) are all aligned, and have a crystal-clear understanding >> of the expectations surrounding kfuncs. >> >> To do that, this patch adds that documentation to the main kfuncs >> documentation page via a new 'kfunc lifecycle expectations' section. The >> patch describes how decisions are made in the kernel regarding whether >> to include, keep, deprecate, or change / remove a kfunc. As described >> very overtly in the patch itself, but likely worth highlighting here: >> >> "kfunc stability" does not mean, nor ever will mean, "BPF APIs may block >> development elsewhere in the kernel". >> >> Rather, the intention and expectation is for kfuncs to be treated like >> EXPORT_SYMBOL_GPL symbols in the kernel. The goal is for kfuncs to be a >> safe and valuable option for maintainers and kfunc developers to extend >> the kernel, without tying anyone's hands, or imposing any kind of >> restrictions on maintainers in the same way that UAPI changes do. > > I think they are still different, kernel modules are still considered as > a part of kernel development, while eBPF code is not that supposed to be > kernel development, at least much further. Treating them alike is > misleading, IMHO. If you read the actual documentation text added to kfuncs.rst this difference is indeed called out. But you're right that "treated like" in the commit message is probably a bit strong. >> In addition to the 'kfunc lifecycle expectations' section, this patch >> also adds documentation for a new KF_DEPRECATED kfunc flag which kfunc >> authors or maintainers can choose to add to kfuncs if and when they >> decide to deprecate them. Note that as described in the patch itself, a >> kfunc need not be deprecated before being changed or removed -- this >> flag is simply provided as an available deprecation mechanism for those >> that want to provide a deprecation story / timeline to their users. >> When necessary, kfuncs may be changed or removed to accommodate changes >> elsewhere in the kernel without any deprecation at all. > > This fundamentally contradicts with Compile-Once-Run-Everywhere > https://facebookmicrosites.github.io/bpf/blog/2020/02/19/bpf-portability-and-co-re.html > Could you add some clarification for this too? Especically how we could > respect CO-RE meanwhile deprecating kfuncs? Well, CO-RE doesn't work for kfuncs, currently, so... :) What do you mean "respect CO-RE", though? CO-RE is a tool to make BPF programs more portable, so not sure how one would "respect" that? > BTW, not related to compatibility, but still kfuncs related confusion, > it also contradicts with Documentation/bpf/bpf_design_QA.rst: > > " > Q: Can BPF functionality such as new program or map types, new > helpers, etc be added out of kernel module code? > > A: NO. > " > > The conntrack kfuncs like bpf_skb_ct_alloc() reside in a kernel > module. Yup, good point, we should update that. I'll send a patch... -Toke
