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Date:   Thu, 29 Oct 2020 19:33:15 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, 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 Thu, Oct 29, 2020 at 5:33 PM Song Liu <songliubraving@...com> wrote:
>
>
>
> > 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?

It will be possible to use for both in-tree and out-of-tree. For
in-tree, this will be integrated into the kernel build process. For
out-of-tree, whoever builds their kernel module will need to invoke
pahole -J with an extra flag pointing to the right vmlinux image (I
haven't looked into the exact details of this integration, maybe there
are already scripts in Linux repo that out-of-tree modules have to
use, in such case we can add this integration there).

Merging all in-tree modules' BTFs into vmlinux's BTF defeats the
purpose of the split BTF and will just increase the size of vmlinux
BTF unnecessarily.

>
> Thanks,
> Song
>
> [...]

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