linux-kernel - Re: [PATCH 2/3] selftests/nolibc: don't pass CC to toplevel Makefile

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Message-ID: <20250721025627.GB1886@1wt.eu>
Date: Mon, 21 Jul 2025 04:56:27 +0200
From: Willy Tarreau <w@....eu>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: Shuah Khan <shuah@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
        Bill Wendling <morbo@...gle.com>,
        Justin Stitt <justinstitt@...gle.com>, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 2/3] selftests/nolibc: don't pass CC to toplevel Makefile

Hi Thomas,

On Sat, Jul 19, 2025 at 05:38:28PM +0200, Thomas Weißschuh wrote:
> The toplevel Makefile is capable of calculating CC from CROSS_COMPILE
> and/or ARCH.
> 
> Stop passing the unnecessary variable.
(...) 
>  # Execute the toplevel kernel Makefile
> -KBUILD_MAKE = $(MAKE) -C $(srctree) ARCH=$(ARCH) CC=$(CC) CROSS_COMPILE=$(CROSS_COMPILE)
> +KBUILD_MAKE = $(MAKE) -C $(srctree) ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE)

Here the goal was not to help the toplevel Makefile figure CC, but rather
to permit the user to override it, and it's also listed in "make help",
and even used in cc-option.

I understnad that you're trying to avoid forcing CC to clang when
building, but in this case, what will CROSS_COMPILE contain ?  My
guess is that you intend to make CROSS_COMPILE point to the gcc-based
toolchain, and have CC point to clang for userland only. Is this the
case ? I think I'd be fine with this, but then we need to make it
explicit in the help message and fix the current one, possibly just
with this:

-	@echo "  nolibc-test       build the executable (uses \$$CC and \$$CROSS_COMPILE)"
+	@echo "  nolibc-test       build the executable (uses \$$CC)"
	@echo "  libc-test         build an executable using the compiler's default libc instead"
	@echo "  run-user          runs the executable under QEMU (uses \$$XARCH, \$$TEST)"
	@echo "  initramfs.cpio    prepare the initramfs archive with nolibc-test"
	@echo "  initramfs         prepare the initramfs tree with nolibc-test"
 	@echo "  defconfig         create a fresh new default config (uses \$$XARCH)"
-	@echo "  kernel            (re)build the kernel (uses \$$XARCH)"
+	@echo "  kernel            (re)build the kernel (uses \$$XARCH, \$$CROSS_COMPILE)"

Thanks,
Willy