linux-kernel - Re: [PATCH] x86: do not expose CONFIG

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Date:	Thu, 29 Jan 2009 00:24:08 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Harvey Harrison <harvey.harrison@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...nel.org>,
	Jaswinder Singh Rajput <jaswinder@...nel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sam Ravnborg <sam@...nborg.org>,
	Jaswinder Singh Rajput <jaswinderrajput@...il.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] x86: do not expose CONFIG_BSWAP to userspace

On Wednesday 28 January 2009, H. Peter Anvin wrote:
> So, one ends up doing something like:
> 
> #ifdef __i486__
> # define __CPU_HAVE_BSWAP
> #endif
> #ifdef __i586__
> # define __CPU_HAVE_BSWAP
> #endif
> 
> ... and so on, and have to keep this up to date with the latest 
> inventions of the gcc people.  *Sob.*

Well, to put this into perspective: The bswap inline assembly was
introduced in Linux-1.3.51 "Greased Weasel", back in 1995 and at
no time it was ever visible to user space, unless the code manually
included <linux/config.h> (which we broke) or defined CONFIG_M486,
CONFIG_X86_BSWAP and/or __KERNEL__, depending on the kernel version.

I take this as a strong indication that user space applications
won't generally expect to get the bswap instruction from including
the kernel headers. For the longest time, we actually had

/* For avoiding bswap on i386 */
#ifdef __KERNEL__
#include <linux/config.h>
#endif

which I read as explicitly using the portable i386 version for
all user space.

	Arnd <><
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