Re: [PATCH bpf-next 05/11] samples: bpf: makefile: use D vars from KBUILD_CFLAGS to handle headers

[Yonghong Song <yhs@fb.com>]

On 9/10/19 11:38 AM, Ivan Khoronzhuk wrote:
> The kernel headers are reused from samples bpf, and autoconf.h is not
> enough to reflect complete arch configuration for clang. But CLANG-bpf
> cmds are sensitive for assembler part taken from linux headers and -D
> vars, usually used in CFLAGS, should be carefully added for each arch.
> For that, for CLANG-bpf, lets filter them only for arm arch as it
> definitely requires __LINUX_ARM_ARCH__ to be set, but ignore for
> others till it's really needed. For arm, -D__LINUX_ARM_ARCH__ is min
> version used as instruction set selector. In another case errors
> like "SMP is not supported" for arm and bunch of other errors are
> issued resulting to incorrect final object.
> 
> Later D_OPTIONS can be used for gcc part.
> ---
>   samples/bpf/Makefile | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> index 8ecc5d0c2d5b..6492b7e65c08 100644
> --- a/samples/bpf/Makefile
> +++ b/samples/bpf/Makefile
> @@ -185,6 +185,15 @@ HOSTLDLIBS_map_perf_test	+= -lrt
>   HOSTLDLIBS_test_overhead	+= -lrt
>   HOSTLDLIBS_xdpsock		+= -pthread
>   
> +# Strip all expet -D options needed to handle linux headers
> +# for arm it's __LINUX_ARM_ARCH__ and potentially others fork vars
> +D_OPTIONS = $(shell echo "$(KBUILD_CFLAGS) " | sed 's/[[:blank:]]/\n/g' | \
> +	sed '/^-D/!d' | tr '\n' ' ')
> +
> +ifeq ($(ARCH), arm)
> +CLANG_EXTRA_CFLAGS := $(D_OPTIONS)
> +endif

Do you need this for native compilation?

so arm64 compilation does not need this?
If only -D__LINUX_ARM_ARCH__ is needed, maybe just
with
    CLANG_EXTRA_CFLAGS := -D__LINUX_ARM_ARCH__
Otherwise, people will wonder whether this is needed for
other architectures. Or just do
    CLANG_EXTRA_CFLAGS := $(D_OPTIONS)
for all cross compilation?

> +
>   # Allows pointing LLC/CLANG to a LLVM backend with bpf support, redefine on cmdline:
>   #  make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
>   LLC ?= llc
>