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Date: Thu, 22 Mar 2007 09:30:30 +0200
From: Avi Kivity <avi@qumranet.com>
Subject: Re: [PATCH] Cleanup: rationalize paravirt wrappers
In-reply-to: <1174531311.2713.86.camel@localhost.localdomain>
To: Rusty Russell <rusty@rustcorp.com.au>
Cc: Andrew Morton <akpm@osdl.org>, Jeremy Fitzhardinge <jeremy@goop.org>,
       lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Andi Kleen <ak@muc.de>
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Rusty Russell wrote:
> +static inline unsigned long long native_read_msr(unsigned int msr, int *err)
> +{
> +	unsigned long long val;
> +
> +	asm volatile("2: rdmsr ; xorl %0,%0\n"
> +		     "1:\n\t"
> +		     ".section .fixup,\"ax\"\n\t"
> +		     "3:  movl %3,%0 ; jmp 1b\n\t"
> +		     ".previous\n\t"
> + 		     ".section __ex_table,\"a\"\n"
> +		     "   .align 4\n\t"
> +		     "   .long 	2b,3b\n\t"
> +		     ".previous"
> +		     : "=r" (*err), "=A" (val)
> +		     : "c" (msr), "i" (-EFAULT));
> +
> +	return val;
> +}
>   


> +#define rdmsr(msr,val1,val2)						\
> +	do {								\
> +		int __err;						\
> +		unsigned long long __val = native_read_msr(msr, &__err);	\
> +		val1 = __val;						\
> +		val2 = __val >> 32;					\
> +	} while(0)
> +
>   

You're silently changing the behavior (as well as the prototype) here,  
rdmsr() used to fail loudly, now it fails silently, with no way for the 
caller to check.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.