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Date: Tue, 03 Feb 2009 21:37:15 +0200
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] don't expose lm bit if kernel is not 64-bit
	capable.
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To: qemu-devel@nongnu.org
Cc: aliguori@us.ibm.com

Glauber Costa wrote:
> If the kernel is not 64-bit capable (even if the host
> machine is) do not expose the lm bit in guest cpuid.
>
>
> diff --git a/target-i386/helper.c b/target-i386/helper.c
> index a28ab93..997e4e1 100644
> --- a/target-i386/helper.c
> +++ b/target-i386/helper.c
> @@ -24,6 +24,7 @@
>  #include <inttypes.h>
>  #include <signal.h>
>  #include <assert.h>
> +#include <sys/utsname.h>
>  
>  #include "cpu.h"
>  #include "exec-all.h"
> @@ -1520,13 +1521,16 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index,
>  
>          if (kvm_enabled()) {
>              uint32_t h_eax, h_edx;
> +            struct utsname utsname;
> +
> +            uname(&utsname);
>  

It would be better to use KVM_GET_SUPPORTED_CPUID for this (and 
similar).  uname() can lie, and in theory kvm can support x86_64 on i386 
kernels (although patches to implement this would be rejected on 
cost/benefit and aesthetic concerns).

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.