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[Xen-devel] Re: [PATCH 1/1] XEN: enlighten, use uninitialized_var(cx)

To: Jiri Slaby <jirislaby@xxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 1/1] XEN: enlighten, use uninitialized_var(cx)
From: Ingo Molnar <mingo@xxxxxxx>
Date: Wed, 26 Aug 2009 08:09:25 +0200
Cc: chrisw@xxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, jeremy@xxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
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* Jiri Slaby <jirislaby@xxxxxxxxx> wrote:

> To avoid a wrong compiler warning, use unitialized_var(cx) in
> xen_init_cpuid_mask.
> cx needn't be initialized for cpuid when ax is 1.
> Signed-off-by: Jiri Slaby <jirislaby@xxxxxxxxx>
> Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>
> Cc: Chris Wright <chrisw@xxxxxxxxxxxx>
> ---
>  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 e90540a..5ab75e2 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -202,7 +202,7 @@ static void xen_cpuid(unsigned int *ax, unsigned int *bx,
>  static __init void xen_init_cpuid_mask(void)
>  {
> -     unsigned int ax, bx, cx, dx;
> +     unsigned int ax, bx, uninitialized_var(cx), dx;

Please dont use uninitialized_var(), it's an unreliable facility: if 
this variable ever grows a real used-without-initialization bug in 
the future, the compiler warning is turned off permanently. It's 
rare but might happen. We are better off with initializing it to 


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