linux-kernel - Re: [PATCH v2 1/2] selftests/lib.mk: handle both LLVM=1 and CC=clang builds

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Message-ID: <20240607171533.GA2636750@thelio-3990X>
Date: Fri, 7 Jun 2024 10:15:33 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: John Hubbard <jhubbard@...dia.com>, Mark Brown <broonie@...nel.org>,
	Shuah Khan <shuah@...nel.org>,
	Muhammad Usama Anjum <usama.anjum@...labora.com>,
	Beau Belgrave <beaub@...ux.microsoft.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Naresh Kamboju <naresh.kamboju@...aro.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Bill Wendling <morbo@...gle.com>, sunliming <sunliming@...inos.cn>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Valentin Obst <kernel@...entinobst.de>,
	linux-kselftest@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v2 1/2] selftests/lib.mk: handle both LLVM=1 and CC=clang
 builds

On Fri, Jun 07, 2024 at 12:12:19PM +0100, Ryan Roberts wrote:
> On 04/06/2024 05:55, John Hubbard wrote:
> > On 6/3/24 3:47 PM, Nathan Chancellor wrote:
> >> Does CC=clang even work for the selftests? lib.mk here uses 'CC :=' so
> >> won't CC=clang get overridden to CC=$(CROSS_COMPILE)gcc?
> >>
> > 
> > I received a report that someone (I forget who or what) was definitely
> > attempting to just set CC=clang. But yes, it definitely doesn't work
> > properly for CROSS_COMPILE.
> 
> This history as I recall, is that I got a bug report [1] stating that:
> 
> # tools/testing/selftests/fchmodat2$ make CC=clang
> 
> and
> 
> # tools/testing/selftests/openat2$ make CC=clang
> 
> were both failing due to the -static-libsan / -static-libasan difference between
> gcc and clang. I attempted to fix that with [2], which used cc-option to
> determine which variant to use. That never got picked up, and John
> coincidentally did a similar fix, but relying on LLVM=1 instead.
> 
> If we are concluding that CC=clang is an invalid way to do this, then I guess we
> should report that back to [1]?
> 
> [1] https://lore.kernel.org/all/202404141807.LgsqXPY5-lkp@intel.com/
> [2]
> https://lore.kernel.org/linux-kselftest/20240417160740.2019530-1-ryan.roberts@arm.com/

I can only speak from the perspective of the main kernel build, as I
don't really know much if anything about the selftests, but I think
CC=clang and LLVM=1 should both be valid. Ideally, they would behave as
they do for the main kernel build (i.e., CC=clang just uses clang for
the compiler and LLVM=1 uses the entire LLVM tools). I realize that for
the selftests, there is probably little use for tools other than the
compiler, assembler, and linker but I think consistency is desirable
here.

I am not at all familiar with the selftests build system, which is
completely different from Kbuild, but I would ack a patch that does
that. Otherwise, I think having a different meaning or handling of
CC=clang or LLVM=1 is the end of the world, but I do think that it
should be documented.

Cheers,
Nathan