linux-kernel - Re: [Xen-devel] [PATCH 8/9] xen/enlighten: Fix compile warnings.

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Date:	Fri, 30 Sep 2011 09:10:51 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Dan Carpenter <error27@...il.com>
Subject: Re: [Xen-devel] [PATCH 8/9] xen/enlighten: Fix compile warnings.

On Thu, 2011-09-29 at 20:52 +0100, Konrad Rzeszutek Wilk wrote:
> linux/arch/x86/xen/enlighten.c: In function ‘xen_start_kernel’:
> linux/arch/x86/xen/enlighten.c:226: warning: ‘cx’ may be used uninitialized in this function
> linux/arch/x86/xen/enlighten.c:240: note: ‘cx’ was declared here

Before 61f4237d5b005767a76f4f3694e68e6f78f392d9 we used to initialise cx
to zero before calling xen_cpuid.

947ccf9c3c30307b774af3666ee74fcd9f47f646 didn't put it back for some
reason.

Regardless I'm not sure how cx can be unused while {a,b,d}x apparently
are not. All four are passed to xen_cpuid(&ax, &bx, &cx, &dx) and even
if gcc were being clever and looking into xen_cpuid all four are in the
output constraints of the real cpuid asm call.

Oh, I see, ax and cx are also in the input side of the asm and ax is
initialised but cx is not and that is the use not the one later in
xen_init_cpuid_mask.

I think that even if cpuid leaf ax=1 happens not to use the subleaf
index in cx we'd be better to initialise cx=0 than use uninitialized_var
here.

Ian.

> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  arch/x86/xen/enlighten.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index 2d69617..9473861 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -237,7 +237,7 @@ static void xen_cpuid(unsigned int *ax, unsigned int *bx,
>  
>  static void __init xen_init_cpuid_mask(void)
>  {
> -	unsigned int ax, bx, cx, dx;
> +	unsigned int ax, bx, uninitialized_var(cx), dx;
>  	unsigned int xsave_mask;
>  
>  	cpuid_leaf1_edx_mask =

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