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. 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? > 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
