linux-kernel - Re: [PATCH v3 bpf-next 05/14] samples: bpf: makefile: use __LINUX_ARM_ARCH_

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Date:   Mon, 16 Sep 2019 13:44:23 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        john fastabend <john.fastabend@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        clang-built-linux@...glegroups.com,
        sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH v3 bpf-next 05/14] samples: bpf: makefile: use
 __LINUX_ARM_ARCH__ selector for arm

On Mon, Sep 16, 2019 at 3:59 AM Ivan Khoronzhuk
<ivan.khoronzhuk@...aro.org> wrote:
>
> For arm, -D__LINUX_ARM_ARCH__=X is min version used as instruction
> set selector and is absolutely required while parsing some parts of
> headers. It's present in KBUILD_CFLAGS but not in autoconf.h, so let's
> retrieve it from and add to programs cflags. In another case errors
> like "SMP is not supported" for armv7 and bunch of other errors are
> issued resulting to incorrect final object.
> ---
>  samples/bpf/Makefile | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> index 8ecc5d0c2d5b..d3c8db3df560 100644
> --- a/samples/bpf/Makefile
> +++ b/samples/bpf/Makefile
> @@ -185,6 +185,16 @@ HOSTLDLIBS_map_perf_test   += -lrt
>  HOSTLDLIBS_test_overhead       += -lrt
>  HOSTLDLIBS_xdpsock             += -pthread
>
> +ifeq ($(ARCH), arm)
> +# Strip all except -D__LINUX_ARM_ARCH__ option needed to handle linux
> +# headers when arm instruction set identification is requested.
> +ARM_ARCH_SELECTOR = $(shell echo "$(KBUILD_CFLAGS) " | \
> +                   sed 's/[[:blank:]]/\n/g' | sed '/^-D__LINUX_ARM_ARCH__/!d')

Does the following work exactly like that without shelling out (and
being arguably simpler)?

ARM_ARCH_SELECTOR = $(filter -D__LINUX_ARM_ARCH__%, $(KBUILD_CFLAGS))

> +
> +CLANG_EXTRA_CFLAGS := $(ARM_ARCH_SELECTOR)
> +KBUILD_HOSTCFLAGS := $(ARM_ARCH_SELECTOR)

Isn't this clearing out previous value of KBUILD_HOSTCFLAGS? Is that
intentional, or it was supposed to be += here?

> +endif
> +
>  # 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
> --
> 2.17.1
>