Re: [PATCH 2/2] kvm/x86: remove unneeded substitute search for missing CPUID entries

[Avi Kivity <avi@redhat.com>]

On 03/30/2011 03:01 PM, Andre Przywara wrote:
> If KVM cannot find an exact match for a requested CPUID leaf, the
> code will try to find the closest match instead of simply confessing
> it's failure. The heuristic is on one hand wrong nowadays,
> since it does not take the KVM CPUID leaves (0x400000xx) into
> account. On the other hand the callers of this function can all deal
> with the no-match situation. So lets remove this code, as it serves
> no purpose.
> This fixes a crash of newer Linux kernels as KVM guests on
> AMD Bulldozer CPUs, where bogus values were returned in response to
> a CPUID intercept.
>
>
> @@ -4959,12 +4959,6 @@ struct kvm_cpuid_entry2 *kvm_find_cpuid_entry(struct kvm_vcpu *vcpu,
>   			best = e;
>   			break;
>   		}
> -		/*
> -		 * Both basic or both extended?
> -		 */
> -		if (((e->function ^ function)&  0x80000000) == 0)
> -			if (!best || e->function>  best->function)
> -				best = e;
>   	}
>   	return best;
>   }


This behaviour is mandated by the spec (looking at the Intel one), 
though it is implemented incorrectly - should always return largest 
basic leaf, and ignore the kvm leaves.

I think the correct behaviour is:

    if (e->function < 10000 && (!best || e->function > best->function))
         best = e;

We probably need a find_exact_cpuid_entry() that returns NULL if it 
doesn't find a match, for internal use.

-- 
error compiling committee.c: too many arguments to function