linux-kernel - Re: Fw: asm-x86/byteorder.h, CONFIG_X86

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Message-Id: <1217090446.5971.57.camel@brick>
Date:	Sat, 26 Jul 2008 09:40:46 -0700
From:	Harvey Harrison <harvey.harrison@...il.com>
To:	Olaf Hering <olaf@...fle.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Fw: asm-x86/byteorder.h, CONFIG_X86_BSWAP leaks to userland

On Sat, 2008-07-26 at 01:39 -0700, Andrew Morton wrote:
> 
> Begin forwarded message:
> 
> Date: Mon, 21 Jul 2008 15:18:46 +0200
> From: Olaf Hering <olaf@...fle.de>
> To: linux-kernel@...r.kernel.org
> Subject: asm-x86/byteorder.h, CONFIG_X86_BSWAP leaks to userland
> What is the purpose of CONFIG_X86_BSWAP in asm-x86/byteorder.h?
> 
> An undefined CONFIG_foo defaults to 0 (I think), so bswap is never used.
> Is this done on purpose, or can the CONFIG_ foo be moved inside
> __KERNEL__ somehow?

I believe it's there to prevent the bswap instruction from being used on
early x86_32 models (i386/i486).  As this will be 0 in userspace it is
effectively never using the bswap instruction for these routines.

I'm not sure if it's time yet to make the bswap ones be exported, as they
would no longer be usable for those early machines.  X86 guys CC:d.

Harvey

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