netdev - Re: [PATCH v3 bpf-next 09/14] samples: bpf: makefile: use own flags but not host when cross compile

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Date:   Wed, 18 Sep 2019 14:29:53 -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 09/14] samples: bpf: makefile: use own flags
 but not host when cross compile

On Wed, Sep 18, 2019 at 3:35 AM Ivan Khoronzhuk
<ivan.khoronzhuk@...aro.org> wrote:
>
> On Tue, Sep 17, 2019 at 04:42:07PM -0700, Andrii Nakryiko wrote:
> >On Mon, Sep 16, 2019 at 3:59 AM Ivan Khoronzhuk
> ><ivan.khoronzhuk@...aro.org> wrote:
> >>
> >> While compile natively, the hosts cflags and ldflags are equal to ones
> >> used from HOSTCFLAGS and HOSTLDFLAGS. When cross compiling it should
> >> have own, used for target arch. While verification, for arm, arm64 and
> >> x86_64 the following flags were used alsways:
> >>
> >> -Wall
> >> -O2
> >> -fomit-frame-pointer
> >> -Wmissing-prototypes
> >> -Wstrict-prototypes
> >>
> >> So, add them as they were verified and used before adding
> >> Makefile.target, but anyway limit it only for cross compile options as
> >> for host can be some configurations when another options can be used,
> >> So, for host arch samples left all as is, it allows to avoid potential
> >> option mistmatches for existent environments.
> >>
> >> Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
> >> ---
> >>  samples/bpf/Makefile | 9 +++++++++
> >>  1 file changed, 9 insertions(+)
> >>
> >> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> >> index 1579cc16a1c2..b5c87a8b8b51 100644
> >> --- a/samples/bpf/Makefile
> >> +++ b/samples/bpf/Makefile
> >> @@ -178,8 +178,17 @@ CLANG_EXTRA_CFLAGS := $(ARM_ARCH_SELECTOR)
> >>  TPROGS_CFLAGS += $(ARM_ARCH_SELECTOR)
> >>  endif
> >>
> >> +ifdef CROSS_COMPILE
> >> +TPROGS_CFLAGS += -Wall
> >> +TPROGS_CFLAGS += -O2
> >
> >Specifying one arg per line seems like overkill, put them in one line?
> Will combine.
>
> >
> >> +TPROGS_CFLAGS += -fomit-frame-pointer
> >
> >Why this one?
> I've explained in commit msg. The logic is to have as much as close options
> to have smiliar binaries. As those options are used before for hosts and kinda
> cross builds - better follow same way.

I'm just asking why omit frame pointers and make it harder to do stuff
like profiling? What performance benefits are we seeking for in BPF
samples?

>
> >
> >> +TPROGS_CFLAGS += -Wmissing-prototypes
> >> +TPROGS_CFLAGS += -Wstrict-prototypes
> >
> >Are these in some way special that we want them in cross-compile mode only?
> >
> >All of those flags seem useful regardless of cross-compilation or not,
> >shouldn't they be common? I'm a bit lost about the intent here...
> They are common but split is needed to expose it at least. Also host for
> different arches can have some own opts already used that shouldn't be present
> for cross, better not mix it for safety.

We want -Wmissing-prototypes and -Wstrict-prototypes for cross-compile
and non-cross-compile cases, right? So let's specify them as common
set of options, instead of relying on KBUILD_HOSTCFLAGS or
HOST_EXTRACFLAGS to have them. Otherwise we'll be getting extra
warnings for just cross-compile case, which is not good. If you are
worrying about having duplicate -W flags, seems like it's handled by
GCC already, so shouldn't be a problem.

>
> >
> >> +else
> >>  TPROGS_LDLIBS := $(KBUILD_HOSTLDLIBS)
> >>  TPROGS_CFLAGS += $(KBUILD_HOSTCFLAGS) $(HOST_EXTRACFLAGS)
> >> +endif
> >> +
> >>  TPROGS_CFLAGS += -I$(objtree)/usr/include
> >>  TPROGS_CFLAGS += -I$(srctree)/tools/lib/bpf/
> >>  TPROGS_CFLAGS += -I$(srctree)/tools/testing/selftests/bpf/
> >> --
> >> 2.17.1
> >>
>
> --
> Regards,
> Ivan Khoronzhuk