Date: Wed, 21 Feb 2018 10:56:24 +0100 From: Arnd Bergmann <arnd@...db.de> To: Masahiro Yamada <yamada.masahiro@...ionext.com> Cc: Ulf Magnusson <ulfalizer@...il.com>, Rich Felker <dalias@...c.org>, Linux-sh list <linux-sh@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Paul Mackerras <paulus@...ba.org>, "H. Peter Anvin" <hpa@...or.com>, sparclinux <sparclinux@...r.kernel.org>, Sam Ravnborg <sam@...nborg.org>, uml-devel <user-mode-linux-devel@...ts.sourceforge.net>, Yoshinori Sato <ysato@...rs.sourceforge.jp>, Jonathan Corbet <corbet@....net>, X86 ML <x86@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Ingo Molnar <mingo@...hat.com>, Emese Revfy <re.emese@...il.com>, Kees Cook <keescook@...omium.org>, Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>, Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>, Jeff Dike <jdike@...toit.com>, user-mode-linux-user@...ts.sourceforge.net, Thomas Gleixner <tglx@...utronix.de>, Michal Marek <michal.lkml@...kovi.net>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Randy Dunlap <rdunlap@...radead.org>, "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Richard Weinberger <richard@....at>, linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>, "David S. Miller" <davem@...emloft.net> Subject: Re: [PATCH 00/23] kconfig: move compiler capability tests to Kconfig On Wed, Feb 21, 2018 at 8:38 AM, Masahiro Yamada <yamada.masahiro@...ionext.com> wrote: > 2018-02-20 0:18 GMT+09:00 Ulf Magnusson <ulfalizer@...il.com>: > >>> >>> I'm not happy that we in one context can reference CONFIG variables >>> directly, but inside the $(call ...) and $(shell ...) needs the $ prefix. >>> But I could not come up with something un-ambigious where this could be avoided. >> >> I think we should be careful about allowing references to config >> symbols. It mixes up the parsing and evaluation phases, since $() is >> expanded during parsing (which I consider a feature and think is >> needed to retain sanity). >> >> Patch 06/23 removes the last existing instance of symbol references in >> strings by getting rid of 'option env'. That's an improvement to me. >> We shouldn't add it back. > > > This is really important design decision, > so I'd like to hear a little more from experts. > > > For example, x86 allows users to choose sub-arch, either 'i386' or 'x86_64'. > > https://github.com/torvalds/linux/blob/v4.16-rc2/arch/x86/Kconfig#L4 > > > > If the user toggles CONFIG_64BIT, > the bi-arch compiler will work in a slightly different mode > (at least, back-end parts) > > So, my question is, is there a case, > > $(cc-option, -m32 -foo) is y, but > $(cc-option, -m64 -foo) is n ? > (or vice versa) > > > If the answer is yes, $(cc-option -foo) would have to be re-calculated > every time CONFIG_64BIT is toggled. > > This is what I'd like to avoid, though. The -m32/-m64 trick (and -mbig-endian/-mlittle-endian on other architectures as well as a couple of other flags) only works if the compiler is configured to support it. In other cases (e.g. big-endian xtensa), the kernel always detects what the compiler does and silently configures itself to match using Makefile logic. On x86, compilers are usually built as bi-arch, but you can build one that only allows one of them. I can see two reasonable ways out: - we don't use $(cc-option -foo) in a case like this, and instead require the user to have a matching toolchain. - we could make the 32/64 selection on x86 a 'choice' statement where each option depends on both the ARCH= variable and the $(cc-option, -m32)/ $(cc-option, -m64) output. Arnd
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