netdev - Re: [PATCH v2 bpf-next 00/11] libbpf: split BTF support

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Date:   Thu, 5 Nov 2020 11:16:19 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.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>,
        Saeed Mahameed <saeed@...nel.org>,
        David Ahern <dsahern@...nel.org>,
        Anton Protopopov <aspsk2@...il.com>
Subject: Re: [PATCH v2 bpf-next 00/11] libbpf: split BTF support

On Thu, Nov 5, 2020 at 1:53 AM Jesper Dangaard Brouer <brouer@...hat.com> wrote:
>
> On Wed, 4 Nov 2020 20:33:50 -0800
> 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.
>
> 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
>