netdev - Re: [PATCH bpf-next v4 01/12] libbpf: skip re-installing headers file if source is older than target

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Date:   Fri, 8 Oct 2021 12:13:15 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Quentin Monnet <quentin@...valent.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v4 01/12] libbpf: skip re-installing headers file
 if source is older than target

On Thu, Oct 7, 2021 at 12:44 PM Quentin Monnet <quentin@...valent.com> wrote:
>
> The "install_headers" target in libbpf's Makefile would unconditionally
> export all API headers to the target directory. When those headers are
> installed to compile another application, this means that make always
> finds newer dependencies for the source files relying on those headers,
> and deduces that the targets should be rebuilt.
>
> Avoid that by making "install_headers" depend on the source header
> files, and (re-)install them only when necessary.
>
> Signed-off-by: Quentin Monnet <quentin@...valent.com>
> ---
>  tools/lib/bpf/Makefile | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
>
> diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
> index 41e4f78dbad5..a92d3b9692a8 100644
> --- a/tools/lib/bpf/Makefile
> +++ b/tools/lib/bpf/Makefile
> @@ -241,15 +241,23 @@ install_lib: all_cmd
>                 $(call do_install_mkdir,$(libdir_SQ)); \
>                 cp -fpR $(LIB_FILE) $(DESTDIR)$(libdir_SQ)
>
> -INSTALL_HEADERS = bpf.h libbpf.h btf.h libbpf_common.h libbpf_legacy.h xsk.h \
> -                 bpf_helpers.h $(BPF_GENERATED) bpf_tracing.h               \
> -                 bpf_endian.h bpf_core_read.h skel_internal.h               \
> -                 libbpf_version.h
> +SRC_HDRS := bpf.h libbpf.h btf.h libbpf_common.h libbpf_legacy.h xsk.h        \
> +           bpf_helpers.h bpf_tracing.h bpf_endian.h bpf_core_read.h           \
> +           skel_internal.h libbpf_version.h
> +GEN_HDRS := $(BPF_GENERATED)
> +INSTALL_SRC_HDRS := $(addprefix $(DESTDIR)$(prefix)/include/bpf/,$(SRC_HDRS))
> +INSTALL_GEN_HDRS := $(addprefix $(DESTDIR)$(prefix)/include/bpf/, \
> +                               $(notdir $(GEN_HDRS)))

I've added INSTALL_PFX := $(DESTDIR)$(prefix)/include/bpf and used it
where possible

> +$(INSTALL_SRC_HDRS): $(DESTDIR)$(prefix)/include/bpf/%.h: %.h
> +       $(call QUIET_INSTALL, $@) \
> +               $(call do_install,$<,$(prefix)/include/bpf,644)
> +$(INSTALL_GEN_HDRS): $(DESTDIR)$(prefix)/include/bpf/%.h: $(OUTPUT)%.h
> +       $(call QUIET_INSTALL, $@) \
> +               $(call do_install,$<,$(prefix)/include/bpf,644)
>
> -install_headers: $(BPF_GENERATED)
> -       $(call QUIET_INSTALL, headers)                                       \
> -               $(foreach hdr,$(INSTALL_HEADERS),                            \
> -                       $(call do_install,$(hdr),$(prefix)/include/bpf,644);)
> +INSTALL_HEADERS := $(INSTALL_SRC_HDRS) $(INSTALL_GEN_HDRS)

I felt like INSTALL_HEADERS just adds one more indirection and
otherwise is not useful, so I dropped it and inlined
$(INSTALL_SRC_HDRS) $(INSTALL_GEN_HDRS) below


> +
> +install_headers: $(BPF_GENERATED) $(INSTALL_HEADERS)
>
>  install_pkgconfig: $(PC_FILE)
>         $(call QUIET_INSTALL, $(PC_FILE)) \
> --
> 2.30.2
>