On Tue, Jan 17, 2023 at 2:04 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > Stanislav Fomichev <sdf@xxxxxxxxxx> writes: > > > On Tue, Jan 17, 2023 at 1:27 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > >> > >> Following up on the discussion at the BPF office hours, this patch adds a > >> description of the (new) concept of "stable kfuncs", which are kfuncs that > >> offer a "more stable" interface than what we have now, but is still not > >> part of UAPI. > >> > >> This is mostly meant as a straw man proposal to focus discussions around > >> stability guarantees. From the discussion, it seemed clear that there were > >> at least some people (myself included) who felt that there needs to be some > >> way to export functionality that we consider "stable" (in the sense of > >> "applications can rely on its continuing existence"). > >> > >> One option is to keep BPF helpers as the stable interface and implement > >> some technical solution for moving functionality from kfuncs to helpers > >> once it has stood the test of time and we're comfortable committing to it > >> as a stable API. Another is to freeze the helper definitions, and instead > >> use kfuncs for this purpose as well, by marking a subset of them as > >> "stable" in some way. Or we can do both and have multiple levels of > >> "stable", I suppose. > >> > >> This patch is an attempt to describe what the "stable kfuncs" idea might > >> look like, as well as to formulate some criteria for what we mean by > >> "stable", and describe an explicit deprecation procedure. Feel free to > >> critique any part of this (including rejecting the notion entirely). > >> > >> Some people mentioned (in the office hours) that should we decide to go in > >> this direction, there's some work that needs to be done in libbpf (and > >> probably the kernel too?) to bring the kfunc developer experience up to par > >> with helpers. Things like exporting kfunc definitions to vmlinux.h (to make > >> them discoverable), and having CO-RE support for using them, etc. I kinda > >> consider that orthogonal to what's described here, but I do think we should > >> fix those issues before implementing the procedures described here. > >> > >> v2: > >> - Incorporate Daniel's changes > >> > >> Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx> > >> --- > >> Documentation/bpf/kfuncs.rst | 87 +++++++++++++++++++++++++++++++++--- > >> 1 file changed, 81 insertions(+), 6 deletions(-) > >> > >> diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst > >> index 9fd7fb539f85..dd40a4ee35f2 100644 > >> --- a/Documentation/bpf/kfuncs.rst > >> +++ b/Documentation/bpf/kfuncs.rst > >> @@ -7,9 +7,9 @@ BPF Kernel Functions (kfuncs) > >> > >> BPF Kernel Functions or more commonly known as kfuncs are functions in the Linux > >> kernel which are exposed for use by BPF programs. Unlike normal BPF helpers, > >> -kfuncs do not have a stable interface and can change from one kernel release to > >> -another. Hence, BPF programs need to be updated in response to changes in the > >> -kernel. > >> +kfuncs by default do not have a stable interface and can change from one kernel > >> +release to another. Hence, BPF programs may need to be updated in response to > >> +changes in the kernel. See :ref:`BPF_kfunc_stability`. > >> > >> 2. Defining a kfunc > >> =================== > >> @@ -223,14 +223,89 @@ type. An example is shown below:: > >> } > >> late_initcall(init_subsystem); > >> > >> -3. Core kfuncs > >> + > >> +.. _BPF_kfunc_stability: > >> + > >> +3. API (in)stability of kfuncs > >> +============================== > >> + > >> +By default, kfuncs exported to BPF programs are considered a kernel-internal > >> +interface that can change between kernel versions. This means that BPF programs > >> +using kfuncs may need to adapt to changes between kernel versions. In the > >> +extreme case that could also include removal of a kfunc. In other words, kfuncs > >> +are _not_ part of the kernel UAPI! Rather, these kfuncs can be thought of as > >> +being similar to internal kernel API functions exported using the > > > > [..] > > > >> +``EXPORT_SYMBOL_GPL`` macro. All new BPF kernel helper-like functionality must > >> +initially start out as kfuncs. > > > > To clarify, as part of this proposal, are we making a decision here > > that we ban new helpers going forward? > > Good question! That is one of the things I'm hoping we can clear up by > this discussing. I don't have a strong opinion on the matter myself, as > long as there is *some* way to mark a subset of helpers/kfuncs as > "stable"... Might be worth it to capitalize in this case to indicate that it's a MUST from the RFC world? (or go with SHOULD otherwise). I'm fine either way. The only thing that stops me from fully embracing MUST is the kfunc requirement on the explicit jit support; I'm not sure why it exists and at this point I'm too afraid to ask. But having MUST here might give us motivation to address the shortcomings... > > (also left some spelling nits below) > > > >> + > >> +3.1 Promotion to "stable" kfuncs > >> +-------------------------------- > >> + > >> +While kfuncs are by default considered unstable as described above, some kfuncs > >> +may warrant a stronger stability guarantee and can be marked as *stable*. The > >> +decision to move a kfunc to *stable* is taken on a case-by-case basis and must > >> +clear a high bar, taking into account the functions' usefulness under > >> +longer-term production deployment without any unforeseen API issues or > >> +limitations. In general, it is not expected that every kfunc will turn into a > >> +stable one - think of it as an exception rather than the norm. > >> + > >> +Those kfuncs which have been promoted to stable are then marked using the > >> +``KF_STABLE`` tag. The process for requesting a kfunc be marked as stable > >> +consists of submitting a patch to the bpf@xxxxxxxxxxxxxxx mailing list adding > >> +the ``KF_STABLE`` tag to that kfunc's definition. The patch description must > >> +include the rationale for why the kfunc should be promoted to stable, including > >> +references to existing production uses, etc. The patch will be considered the > >> +same was as any other patch, and ultimately the decision on whether a kfunc > > > > nit: most likely s/same was/same way/ here? > > Yup! > > >> +should be promoted to stable is taken by the BPF maintainers. > >> + > >> +Stable kfuncs provide the following stability guarantees: > >> + > >> +1. Stable kfuncs will not change their function signature or functionality in a > >> + way that may cause incompatibilities for BPF programs calling the function. > >> + > >> +2. The BPF community will make every reasonable effort to keep stable kfuncs > >> + around as long as they continue to be useful to real-world BPF applications. > >> + > >> +3. Should a stable kfunc turn out to be no longer useful, the BPF community may > >> + decide to eventually remove it. In this case, before being removed that kfunc > >> + will go through a deprecation procedure as outlined below. > >> + > >> +3.2 Deprecation of kfuncs > >> +------------------------- > >> + > >> +As described above, the community will make every reasonable effort to keep > >> +kfuncs available through future kernel versions once they are marked as stable. > >> +However, it may happen case that BPF development moves in an unforeseen > > > > 'may happen case' -> 'may happen in case' ? > > Think I actually meant to drop 'case' entirely; thanks for spotting! > > -Toke >
