linux-kernel - Re: [PATCH v2] tools/build: Add clang cross-compilation flags to feature detection

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Date: Mon, 29 Apr 2024 18:11:45 +0100
From: Jean-Philippe Brucker <jean-philippe@...aro.org>
To: Björn Töpel <bjorn@...nel.org>
Cc: Jiri Olsa <olsajiri@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-kernel@...r.kernel.org,
	Björn Töpel <bjorn@...osinc.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Tom Rix <trix@...hat.com>, bpf@...r.kernel.org,
	Anders Roxell <anders.roxell@...aro.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v2] tools/build: Add clang cross-compilation flags to
 feature detection

On Fri, Apr 26, 2024 at 12:31:17PM +0200, Björn Töpel wrote:
> Jiri Olsa <olsajiri@...il.com> writes:
> 
> > On Thu, Nov 02, 2023 at 11:32:52AM +0100, Björn Töpel wrote:
> >> From: Björn Töpel <bjorn@...osinc.com>
> >> 
> >> When a tool cross-build has LLVM=1 set, the clang cross-compilation
> >> flags are not passed to the feature detection build system. This
> >> results in the host's features are detected instead of the targets.
> >> 
> >> E.g, triggering a cross-build of bpftool:
> >> 
> >>   cd tools/bpf/bpftool
> >>   make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- LLVM=1
> >> 
> >> would report the host's, and not the target's features.
> >> 
> >> Correct the issue by passing the CLANG_CROSS_FLAGS variable to the
> >> feature detection makefile.
> >> 
> >> Fixes: cebdb7374577 ("tools: Help cross-building with clang")
> >> Signed-off-by: Björn Töpel <bjorn@...osinc.com>
> >
> > Acked-by: Jiri Olsa <jolsa@...nel.org>
> 
> Waking up the dead!
> 
> Arnaldo, Jean-Philippe: I'm still stung by what this patch fixes. LMK
> what you need from me/this patch to pick it up.

I guess the problem is these files don't have a specific tree. Since you
mention BPF maybe it should go through the BPF tree, in which case you
could resend to the tools/bpf maintainers (and "PATCH bpf" subject prefix)

FWIW the change looks good to me:

Reviewed-by: Jean-Philippe Brucker <jean-philippe@...aro.org>