linux-kernel - Re: [patch] use __asm__ and __volatile__ in asm-x86

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Message-ID: <467635D8.5060202@zytor.com>
Date:	Mon, 18 Jun 2007 00:35:52 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mike Frysinger <vapier@...too.org>
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, ak@...e.de
Subject: Re: [patch] use __asm__ and __volatile__ in asm-x86_64/msr.h

Mike Frysinger wrote:
> Most of the macros in msr.h already use __asm__ and __volatile__, this patch
> brings the rest into line.  This is needed since these get exported into
> userspace (need to use strict __asm__ vs asm when gcc compiles with GNU
> extensions turned off).
> ---
> diff --git a/include/asm-x86_64/msr.h b/include/asm-x86_64/msr.h
> index d5c55b8..ed15d2b 100644
> --- a/include/asm-x86_64/msr.h
> +++ b/include/asm-x86_64/msr.h
> @@ -33,7 +33,7 @@
>  
>  /* wrmsr with exception handling */
>  #define wrmsr_safe(msr,a,b) ({ int ret__;			\
> -	asm volatile("2: wrmsr ; xorl %0,%0\n"			\
> +	__asm__ __volatile__("2: wrmsr ; xorl %0,%0\n"			\
>  		     "1:\n\t"					\
>  		     ".section .fixup,\"ax\"\n\t"		\
>  		     "3:  movl %4,%0 ; jmp 1b\n\t"		\
> @@ -50,7 +50,7 @@
>  
>  #define rdmsr_safe(msr,a,b) \
>  	({ int ret__;						\
> -	  asm volatile ("1:       rdmsr\n"			\
> +	  __asm__ __volatile__ ("1:       rdmsr\n"			\
>                        "2:\n"					\
>                        ".section .fixup,\"ax\"\n"		\
>                        "3:       movl %4,%0\n"			\

NAK.

This is wrong for two reasons:

- MSRs are not accessible in user space.
- .section .fixup is not applicable to user space.

Anything MSR-related should be protected by __KERNEL__ instead.

This NAK doesn't apply to rdtsc* and rdpmc* changes, however, as those
instructions are legitimate in userspace.

	-hpa
-
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