Re: [PATCH] kvm_para_available() should check hypervisor bit before accessing hypervisor cpuid leaf.

[Davidlohr Bueso <dave@gnu.org>]

On Mon, 2012-04-30 at 14:45 +0300, Gleb Natapov wrote:
> This couid range does not exist on real HW and Intel spec says that
> "Information returned for highest basic information leaf" will be
> returned. Not very well defined. 
> 
> Signed-off-by: Gleb Natapov <gleb@redhat.com>
> diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
> index 99c4bbe..a7a7a94 100644
> --- a/arch/x86/include/asm/kvm_para.h
> +++ b/arch/x86/include/asm/kvm_para.h
> @@ -178,14 +178,16 @@ static inline int kvm_para_available(void)
>  	unsigned int eax, ebx, ecx, edx;
>  	char signature[13];
>  
> -	cpuid(KVM_CPUID_SIGNATURE, &eax, &ebx, &ecx, &edx);
> -	memcpy(signature + 0, &ebx, 4);
> -	memcpy(signature + 4, &ecx, 4);
> -	memcpy(signature + 8, &edx, 4);
> -	signature[12] = 0;
> -
> -	if (strcmp(signature, "KVMKVMKVM") == 0)
> -		return 1;
> +	if (cpu_has_hypervisor) {
> +		cpuid(KVM_CPUID_SIGNATURE, &eax, &ebx, &ecx, &edx);
> +		memcpy(signature + 0, &ebx, 4);
> +		memcpy(signature + 4, &ecx, 4);
> +		memcpy(signature + 8, &edx, 4);
> +		signature[12] = 0;
> +
> +		if (strcmp(signature, "KVMKVMKVM") == 0)
> +			return 1;
> +	}
>  
>  	return 0;
>  }

Wouldn't this be better?

--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -170,6 +170,9 @@ static inline int kvm_para_available(void)
        unsigned int eax, ebx, ecx, edx;
        char signature[13];
 
+       if (!cpu_has_hypervisor)
+               return 0;
+
        cpuid(KVM_CPUID_SIGNATURE, &eax, &ebx, &ecx, &edx);
        memcpy(signature + 0, &ebx, 4);
        memcpy(signature + 4, &ecx, 4);