linux-kernel - Re: [PATCH bpf-next 3/3] bpf: iterators: build and use lightweight bootstrap version of bpftool

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Date:   Wed, 13 Jul 2022 11:55:09 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Pu Lehui <pulehui@...wei.com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Quentin Monnet <quentin@...valent.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Subject: Re: [PATCH bpf-next 3/3] bpf: iterators: build and use lightweight
 bootstrap version of bpftool

On Mon, Jul 11, 2022 at 7:37 PM Pu Lehui <pulehui@...wei.com> wrote:
>
> kernel/bpf/preload/iterators use bpftool for vmlinux.h, skeleton, and
> static linking only. So we can use lightweight bootstrap version of
> bpftool to handle these, and it will be faster.
>
> Signed-off-by: Pu Lehui <pulehui@...wei.com>
> Suggested-by: Andrii Nakryiko <andrii@...nel.org>
> ---
>  kernel/bpf/preload/iterators/Makefile | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/bpf/preload/iterators/Makefile b/kernel/bpf/preload/iterators/Makefile
> index bfe24f8c5a20..cf5f39f95fed 100644
> --- a/kernel/bpf/preload/iterators/Makefile
> +++ b/kernel/bpf/preload/iterators/Makefile
> @@ -9,7 +9,7 @@ LLVM_STRIP ?= llvm-strip
>  TOOLS_PATH := $(abspath ../../../../tools)
>  BPFTOOL_SRC := $(TOOLS_PATH)/bpf/bpftool
>  BPFTOOL_OUTPUT := $(abs_out)/bpftool
> -DEFAULT_BPFTOOL := $(OUTPUT)/sbin/bpftool
> +DEFAULT_BPFTOOL := $(BPFTOOL_OUTPUT)/bootstrap/bpftool
>  BPFTOOL ?= $(DEFAULT_BPFTOOL)
>
>  LIBBPF_SRC := $(TOOLS_PATH)/lib/bpf
> @@ -61,9 +61,14 @@ $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OU
>                     OUTPUT=$(abspath $(dir $@))/ prefix=                       \
>                     DESTDIR=$(LIBBPF_DESTDIR) $(abspath $@) install_headers
>
> +ifeq ($(CROSS_COMPILE),)
>  $(DEFAULT_BPFTOOL): $(BPFOBJ) | $(BPFTOOL_OUTPUT)
>         $(Q)$(MAKE) $(submake_extras) -C $(BPFTOOL_SRC)                        \
>                     OUTPUT=$(BPFTOOL_OUTPUT)/                                  \
> -                   LIBBPF_OUTPUT=$(LIBBPF_OUTPUT)/                            \
> -                   LIBBPF_DESTDIR=$(LIBBPF_DESTDIR)/                          \
> -                   prefix= DESTDIR=$(abs_out)/ install-bin
> +                   LIBBPF_BOOTSTRAP_OUTPUT=$(LIBBPF_OUTPUT)/                  \
> +                   LIBBPF_BOOTSTRAP_DESTDIR=$(LIBBPF_DESTDIR)/ bootstrap
> +else
> +$(DEFAULT_BPFTOOL): | $(BPFTOOL_OUTPUT)
> +       $(Q)$(MAKE) $(submake_extras) -C $(BPFTOOL_SRC)                        \
> +                   OUTPUT=$(BPFTOOL_OUTPUT)/ bootstrap
> +endif

another idea (related to my two previous comments for this patch set),
maybe we can teach bpftool's Makefile to reuse LIBBPF_OUTPUT as
LIBBPF_BOOTSTRAP_OUTPUT, if there is no CROSS_COMPILE? Then we can
keep iterators/Makefile, samples/bpf/Makefile and runqslower/Makefile
simpler and ignorant of CROSS_COMPILE, but still get the benefit of
not rebuilding libbpf unnecessarily in non-cross-compile mode?

> --
> 2.25.1
>