netdev - Re: [PATCH bpf-next 00/11] libbpf: split BTF support

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Message-ID: <4427E5BD-5EBF-47E8-B7F6-9255BEAE2D53@fb.com>
Date:   Fri, 30 Oct 2020 00:33:04 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andrii@...nel.org>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        "Alexei Starovoitov" <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 00/11] libbpf: split BTF support



> On Oct 28, 2020, at 5:58 PM, Andrii Nakryiko <andrii@...nel.org> 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.

Some high level questions. Do we plan to use split BTF for in-tree modules
(those built together with the kernel) or out-of-tree modules (those built 
separately)? If it is for in-tree modules, is it possible to build split BTF
into vmlinux BTF? 

Thanks,
Song

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