linux-kernel - Re: PROBLEM: compilation issue, inline assembly arch/x86/kvm/emulate.c fails at -O0

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Date:	Wed, 21 Nov 2012 13:35:03 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Blower, Melanie" <melanie.blower@...el.com>
CC:	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"avi@...hat.com" <avi@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: PROBLEM: compilation issue, inline assembly arch/x86/kvm/emulate.c
 fails at -O0

On 11/14/2012 11:45 AM, Blower, Melanie wrote:
> [1.] gcc -O0 assembly arch/x86/kvm/emulate.c gets compilation failure -- incorrect register restrictions
> [2.] Full description of the problem/report:
> I'm trying to compile this file at -O0, but gcc chokes in register allocation at the inline assembly.
> 
> In the ordinary Linux build, this file compiles with gcc at -O2, without compilation errors.
> 
> At -O0, gcc chokes with this message: 
> gcc  -w -c ./emulateE.c               // (using preprocessed file)
> ./emulateE.c: In function `em_mul_ex':
> ./emulateE.c:1918:5: error: can't find a register in class `AREG' while reloading `asm'
> ./emulateE.c:1918:5: error: `asm' operand has impossible constraints
> 
> Explanation:
> The file contains an inline asm of a kind:
> 
>     __asm__ __volatile__ ( " ..... " :
> 
>  "=m" ((ctxt)->eflags), "=&r" (_tmp), "+a" (*rax), "+d" (*rdx), "+qm"(ex) :
>  "i" (11), "m" ((ctxt)->src . val), "a" (*rax), "d" (*rdx)); 
> 
> Note that "+a" in inputs already means that eax is the return value. An then "a" is used as an output constraint too.
> 

Hi Melanie,

Can you test the attached patch?

	-hpa



View attachment "0001-x86-kvm-Remove-incorrect-redundant-assembly-constrai.patch" of type "text/x-patch" (1426 bytes)