Re: [PATCH bpf-next 07/11] samples: bpf: add makefile.prog for separate CC build

[Yonghong Song <yhs@fb.com>]

On 9/10/19 11:38 AM, Ivan Khoronzhuk wrote:
> The makefile.prog is added only, will be used in sample/bpf/Makefile
> later in order to switch cross-compiling on CC from HOSTCC.
> 
> The HOSTCC is supposed to build binaries and tools running on the host
> afterwards, in order to simplify build or so, like "fixdep" or else.
> In case of cross compiling "fixdep" is executed on host when the rest
> samples should run on target arch. In order to build binaries for
> target arch with CC and tools running on host with HOSTCC, lets add
> Makefile.prog for simplicity, having definition and routines similar
> to ones, used in script/Makefile.host. This allows later add
> cross-compilation to samples/bpf with minimum changes.

So this is really Makefile.host or Makefile.user, right?
In BPF, 'prog' can refers to user prog or bpf prog.
To avoid ambiguity, maybe Makefile.host?

> 
> Makefile.prog contains only stuff needed for samples/bpf, potentially
> can be reused and extended for other prog sets later and now needed

What do you mean 'extended for other prog sets'? I am wondering whether
we could just include 'scripts/Makefile.host'? How hard it is?

> only for unblocking tricky samples/bpf cross compilation.
> 
> Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
> ---
>   samples/bpf/Makefile.prog | 77 +++++++++++++++++++++++++++++++++++++++
>   1 file changed, 77 insertions(+)
>   create mode 100644 samples/bpf/Makefile.prog
> 
> diff --git a/samples/bpf/Makefile.prog b/samples/bpf/Makefile.prog
> new file mode 100644
> index 000000000000..3781999b9193
> --- /dev/null
> +++ b/samples/bpf/Makefile.prog
> @@ -0,0 +1,77 @@
> +# SPDX-License-Identifier: GPL-2.0
> +# ==========================================================================
> +# Building binaries on the host system
> +# Binaries are not used during the compilation of the kernel, and intendent
> +# to be build for target board, target board can be host ofc. Added to build
> +# binaries to run not on host system.
> +#
> +# Only C is supported, but can be extended for C++.

The above comment is not needed.
samples/bpf/ only have C now. I am wondering whether your below scripts 
can be simplified, e.g., removing cxxobjs.

> +#
> +# Sample syntax (see Documentation/kbuild/makefiles.rst for reference)
> +# progs-y := xsk_example
> +# Will compile xdpsock_example.c and create an executable named xsk_example
> +#
> +# progs-y    := xdpsock
> +# xdpsock-objs := xdpsock_1.o xdpsock_2.o
> +# Will compile xdpsock_1.c and xdpsock_2.c, and then link the executable
> +# xdpsock, based on xdpsock_1.o and xdpsock_2.o
> +#
> +# Inherited from scripts/Makefile.host
> +#
> +__progs := $(sort $(progs-y))
> +
> +# C code
> +# Executables compiled from a single .c file
> +prog-csingle	:= $(foreach m,$(__progs), \
> +			$(if $($(m)-objs)$($(m)-cxxobjs),,$(m)))
> +
> +# C executables linked based on several .o files
> +prog-cmulti	:= $(foreach m,$(__progs),\
> +		   $(if $($(m)-cxxobjs),,$(if $($(m)-objs),$(m))))
> +
> +# Object (.o) files compiled from .c files
> +prog-cobjs	:= $(sort $(foreach m,$(__progs),$($(m)-objs)))
> +
> +prog-csingle	:= $(addprefix $(obj)/,$(prog-csingle))
> +prog-cmulti	:= $(addprefix $(obj)/,$(prog-cmulti))
> +prog-cobjs	:= $(addprefix $(obj)/,$(prog-cobjs))
> +
> +#####
> +# Handle options to gcc. Support building with separate output directory
> +
> +_progc_flags   = $(PROGS_CFLAGS) \
> +                 $(PROGCFLAGS_$(basetarget).o)
> +
> +# $(objtree)/$(obj) for including generated headers from checkin source files
> +ifeq ($(KBUILD_EXTMOD),)
> +ifdef building_out_of_srctree
> +_progc_flags   += -I $(objtree)/$(obj)
> +endif
> +endif
> +
> +progc_flags    = -Wp,-MD,$(depfile) $(_progc_flags)
> +
> +# Create executable from a single .c file
> +# prog-csingle -> Executable
> +quiet_cmd_prog-csingle 	= CC  $@
> +      cmd_prog-csingle	= $(CC) $(progc_flags) $(PROGS_LDFLAGS) -o $@ $< \
> +		$(PROGS_LDLIBS) $(PROGLDLIBS_$(@F))
> +$(prog-csingle): $(obj)/%: $(src)/%.c FORCE
> +	$(call if_changed_dep,prog-csingle)
> +
> +# Link an executable based on list of .o files, all plain c
> +# prog-cmulti -> executable
> +quiet_cmd_prog-cmulti	= LD  $@
> +      cmd_prog-cmulti	= $(CC) $(progc_flags) $(PROGS_LDFLAGS) -o $@ \
> +			  $(addprefix $(obj)/,$($(@F)-objs)) \
> +			  $(PROGS_LDLIBS) $(PROGLDLIBS_$(@F))
> +$(prog-cmulti): $(prog-cobjs) FORCE
> +	$(call if_changed,prog-cmulti)
> +$(call multi_depend, $(prog-cmulti), , -objs)
> +
> +# Create .o file from a single .c file
> +# prog-cobjs -> .o
> +quiet_cmd_prog-cobjs	= CC  $@
> +      cmd_prog-cobjs	= $(CC) $(progc_flags) -c -o $@ $<
> +$(prog-cobjs): $(obj)/%.o: $(src)/%.c FORCE
> +	$(call if_changed_dep,prog-cobjs)
>