linux-kernel - Re: [RFC] perf build: Allow passing extra Clang flags via EXTRA_CLANG

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Message-ID: <aPWNaOqocEPgAJcq@google.com>
Date: Mon, 20 Oct 2025 10:16:24 +0900
From: Namhyung Kim <namhyung@...nel.org>
To: hupu <hupu.gm@...il.com>
Cc: Leo Yan <leo.yan@....com>, Ian Rogers <irogers@...gle.com>,
	peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
	mark.rutland@....com, alexander.shishkin@...ux.intel.com,
	jolsa@...nel.org, adrian.hunter@...el.com, nathan@...nel.org,
	nick.desaulniers+lkml@...il.com, morbo@...gle.com,
	justinstitt@...gle.com, linux-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org
Subject: Re: [RFC] perf build: Allow passing extra Clang flags via
 EXTRA_CLANG_FLAGS

On Sun, Oct 19, 2025 at 03:57:27PM +0800, hupu wrote:
> Hi Namhyung,
> Thanks for your reply.
> 
> On Sun, Oct 19, 2025 at 10:30 AM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > Hello,
> >
> > On Sat, Oct 18, 2025 at 06:05:03PM +0800, hupu wrote:
> > > Hi Leo, Ian, and fellow maintainers.
> > >
> > > On Wed, Oct 15, 2025 at 7:47 PM hupu <hupu.gm@...il.com> wrote:
> > > >
> > > > Hi Leo,
> > > > Thank you for your reply.
> > > >
> > > > On Wed, Oct 15, 2025 at 5:30 PM Leo Yan <leo.yan@....com> wrote:
> > > > >
> > > > > Have you installed the GCC cross packages ?
> > > > >
> > > > >  $ sudo apt-get install gcc-aarch64-linux-gnu g++-aarch64-linux-gnu
> > > > >  $ sudo apt-get install libc6-dev-aarch64-cross linux-libc-dev-aarch64-cross
> > > > >  $ sudo apt-get install libc6-dev-arm64-cross linux-libc-dev-arm64-cross
> > > > >
> > > > > My understanding is arm64 cross compilation tries to find headers in the
> > > > > path /usr/aarch64-linux-gnu/include/ (I confirmed this on Ubuntu/Debian
> > > > > distros).  After install GCC cross packages, the headers should appear
> > > > > in the folder.
> > > > >
> > > >
> > > > I hadn’t installed the packages you mentioned earlier, but after
> > > > running the installation commands you provided, I was indeed able to
> > > > successfully build perf.
> > > >
> > > > In fact, I’m currently working on creating an SDK package, which
> > > > includes a cross-toolchain that I built myself using crosstool-NG. My
> > > > initial idea was to install certain third-party libraries (such as the
> > > > packages you mentioned) into the cross-toolchain’s sysroot directory.
> > > > With this approach, even when developing on different host machines,
> > > > we could simply specify the header search path (pointing to the
> > > > cross-toolchain’s sysroot directory) during compilation, and the build
> > > > should succeed without requiring any additional package installation
> > > > on the system.
> > > >
> > > > Based on this, I think allowing users to extend some options via
> > > > EXTRA_CLANG_FLAGS could be a flexible way to handle such cases.
> > > > However, this is just my personal thought and might not be entirely
> > > > correct, so I’d like to hear your advice.
> > > >
> > >
> > >
> > > Although installing the GCC cross packages allows me to build perf
> > > successfully, I still prefer to be able to pass additional Clang flags
> > > via EXTRA_CLANG_FLAGS, as this approach feels more flexible to me.
> > >
> > > I look forward to continuing the discussion on this topic.
> >
> > Doesn't EXTRA_CFLAGS work for you?
> >
> 
> 
> I’ve tried using EXTRA_CFLAGS and confirm that it doesn’t work. When I
> build perf with the following command, it still reports errors:
>   make perf ARCH=$ARCH CROSS_COMPILE=$CROSS_COMPILE
> EXTRA_CFLAGS="--sysroot=$SYSROOT"

Thanks for checking this.

> 
> In fact, by checking the code in tools/perf/Makefile.perf, we can see
> that the EXTRA_CFLAGS variable is not passed to Clang at all:
> 
> $(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h
> $(QUIET_CLANG)$(CLANG) -g -O2 -fno-stack-protector --target=bpf \
>   $(CLANG_OPTIONS) $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
>   -include $(OUTPUT)PERF-VERSION-FILE -include util/bpf_skel/perf_version.h \
>   -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@
> 
> 
> As shown above, EXTRA_CFLAGS only affects the GCC cross-compilation
> process and has no impact on the Clang build process.
> That’s why I proposed introducing an EXTRA_CLANG_FLAGS variable to
> provide a way to control Clang’s compilation behavior more flexibly.

One thing I care is that we can set clang as a default compiler using CC
variable then meaning of EXTRA_CFLAGS and EXTRA_CLANG_FLAGS may be
confusing.  I guess EXTRA_BPF_FLAGS is clearer that we want to pass
something when it builds BPF programs.

Thanks,
Namhyung