On Thu, Nov 5, 2020 at 1:53 AM Jesper Dangaard Brouer <brouer@xxxxxxxxxx> wrote: > > On Wed, 4 Nov 2020 20:33:50 -0800 > Andrii Nakryiko <andrii@xxxxxxxxxx> wrote: > > > This patch set adds support for generating and deduplicating split BTF. This > > is an enhancement to the BTF, which allows to designate one BTF as the "base > > BTF" (e.g., vmlinux BTF), and one or more other BTFs as "split BTF" (e.g., > > kernel module BTF), which are building upon and extending base BTF with extra > > types and strings. > > > > Once loaded, split BTF appears as a single unified BTF superset of base BTF, > > with continuous and transparent numbering scheme. This allows all the existing > > users of BTF to work correctly and stay agnostic to the base/split BTFs > > composition. The only difference is in how to instantiate split BTF: it > > requires base BTF to be alread instantiated and passed to btf__new_xxx_split() > > or btf__parse_xxx_split() "constructors" explicitly. > > > > This split approach is necessary if we are to have a reasonably-sized kernel > > module BTFs. By deduping each kernel module's BTF individually, resulting > > module BTFs contain copies of a lot of kernel types that are already present > > in vmlinux BTF. Even those single copies result in a big BTF size bloat. On my > > kernel configuration with 700 modules built, non-split BTF approach results in > > 115MBs of BTFs across all modules. With split BTF deduplication approach, > > total size is down to 5.2MBs total, which is on part with vmlinux BTF (at > > around 4MBs). This seems reasonable and practical. As to why we'd need kernel > > module BTFs, that should be pretty obvious to anyone using BPF at this point, > > as it allows all the BTF-powered features to be used with kernel modules: > > tp_btf, fentry/fexit/fmod_ret, lsm, bpf_iter, etc. > > I love to see this work going forward. > Thanks. > My/Our (+Saeed +Ahern) use-case is for NIC-driver kernel modules. I > want drivers to define a BTF struct that describe a meta-data area that > can be consumed/used by XDP, also available during xdp_frame to SKB > transition, which happens in net-core. So, I hope BTF-IDs are also > "available" from core kernel code? I'll probably need a more specific example to understand what exactly you are asking and how you see everything working together, sorry. If you are asking about support for using BTF_ID_LIST() macro in a kernel module, then right now we don't call resolve_btfids on modules, so it's not supported there yet. It's trivial to add, but we'll probably need to teach resolve_btfids to understand split BTF. We can do that separately after the basic "infra" lands, though. > > > > This patch set is a pre-requisite to adding split BTF support to pahole, which > > is a prerequisite to integrating split BTF into the Linux kernel build setup > > to generate BTF for kernel modules. The latter will come as a follow-up patch > > series once this series makes it to the libbpf and pahole makes use of it. > > > > Patch #4 introduces necessary basic support for split BTF into libbpf APIs. > > Patch #8 implements minimal changes to BTF dedup algorithm to allow > > deduplicating split BTFs. Patch #11 adds extra -B flag to bpftool to allow to > > specify the path to base BTF for cases when one wants to dump or inspect split > > BTF. All the rest are refactorings, clean ups, bug fixes and selftests. > > > > v1->v2: > > - addressed Song's feedback. > -- > Best regards, > Jesper Dangaard Brouer > MSc.CS, Principal Kernel Engineer at Red Hat > LinkedIn: http://www.linkedin.com/in/brouer >
