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. > > 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? 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. Thanks!
