linux-kernel - Re: [PATCH] KVM: 'asm' operand has impossible constraints

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Date:	Sat, 27 Jan 2007 11:05:53 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	caglar@...dus.org.tr
CC:	lkml <linux-kernel@...r.kernel.org>,
	kvm-devel@...ts.sourceforge.net
Subject: Re: [PATCH] KVM: 'asm' operand has impossible constraints

S.Çağlar Onur wrote:
> Hi;
>
> -rc6 fails with latest gcc 4.2 snapshot as following;
>
> CC [M]  drivers/kvm/svm.o
> drivers/kvm/svm.c:206: warning: 'inject_db' defined but not used
> drivers/kvm/svm.c: In function 'svm_vcpu_run':
> drivers/kvm/kvm.h:560: error: 'asm' operand has impossible constraints
> make[2]: *** [drivers/kvm/svm.o] Error 1
> make[1]: *** [drivers/kvm] Error 2
> make: *** [drivers] Error 2
>
> And according to this thread http://lkml.org/lkml/2006/11/11/129, solution is 
> converting g to rm, patch follows
>
>   

The patch looks correct, but I don't understand the gcc error message.  
Are we sure this isn't a gcc 4.2 bug?

"g" appears to be equivalent to "rmi", if "i" is impossible, gcc is free 
to use "r" or "m", no?


> Signed-off-by: S.Çağlar Onur <caglar@...dus.org.tr>
>
> Index: linux-2.6/drivers/kvm/kvm.h
> ===================================================================
> --- linux-2.6.orig/drivers/kvm/kvm.h	2007-01-26 01:38:35.000000000 +0200
> +++ linux-2.6/drivers/kvm/kvm.h	2007-01-26 01:37:48.000000000 +0200
> @@ -557,7 +557,7 @@
>  #ifndef load_ldt
>  static inline void load_ldt(u16 sel)
>  {
> -	asm ("lldt %0" : : "g"(sel));
> +	asm ("lldt %0" : : "rm"(sel));
>  }
>  #endif
>
>
>   


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

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