linux-kernel - Re: [PATCH 2/8] um: Do not use SUBARCH

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Date:	Thu, 26 Sep 2013 16:10:19 +0530
From:	Ramkumar Ramachandra <artagnon@...il.com>
To:	Richard Weinberger <richard@....at>
Cc:	linux-arch@...r.kernel.org, Michal Marek <mmarek@...e.cz>,
	geert@...ux-m68k.org, ralf@...ux-mips.org, lethal@...ux-sh.org,
	Jeff Dike <jdike@...toit.com>, gxt@...c.pku.edu.cn,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kbuild@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	linux-m68k@...ts.linux-m68k.org, linux-mips@...ux-mips.org,
	linux-sh@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net
Subject: Re: [PATCH 2/8] um: Do not use SUBARCH

Richard Weinberger wrote:
> This patch is based on: https://lkml.org/lkml/2013/7/4/396

This is the original patch I sent across in July.

> diff --git a/arch/um/Makefile b/arch/um/Makefile
> index 133f7de..5bc7892 100644
> --- a/arch/um/Makefile
> +++ b/arch/um/Makefile
> @@ -8,6 +8,8 @@
>
>  ARCH_DIR := arch/um
>  OS := $(shell uname -s)
> +OS_ARCH := $(shell uname -m)
> +
>  # We require bash because the vmlinux link and loader script cpp use bash
>  # features.
>  SHELL := /bin/bash
> @@ -20,15 +22,14 @@ core-y                      += $(ARCH_DIR)/kernel/          \
>
>  MODE_INCLUDE   += -I$(srctree)/$(ARCH_DIR)/include/shared/skas
>
> -HEADER_ARCH    := $(SUBARCH)
> -
> -# Additional ARCH settings for x86
> -ifeq ($(SUBARCH),i386)
> -        HEADER_ARCH := x86
> -endif
> -ifeq ($(SUBARCH),x86_64)
> -        HEADER_ARCH := x86
> -       KBUILD_CFLAGS += -mcmodel=large
> +# Currently we support only i386 and x86_64, if you port UML to another arch
> +# add another if branch...
> +ifeq ($(OS_ARCH),x86_64)
> +       HEADER_ARCH := x86
> +       KBUILD_DEFCONFIG := x86_64_defconfig
> +else
> +       HEADER_ARCH := x86
> +       KBUILD_DEFCONFIG := i386_defconfig
>  endif

I honestly don't get why this approach is superior to the original
one. In the original, I could set SUBARCH to whatever target
architecture and attempt to build user-mode Linux. Fact that it is
only implemented for i386 and x86_64 aside, keeping a SUBARCH means
that it's possible to build a 32-bit kernel on a 64-bit machine and
vice-versa. If you want stuff to work automagically (ie. in the 90%
case), you have to shell out to uname -m to figure out the host's real
architecture. Which both versions do sufficiently well.

Forget all that. What matters is that upstream is still broken, and
users are suffering. Despite a reasonable fix being submitted in July.
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