Re: [PATCH 2/2] kvm/x86: move and fix substitue search for missing CPUID entries

[Andre Przywara <andre.przywara@amd.com>]

Avi Kivity wrote:
> On 03/31/2011 03:13 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 implementation was meant to satisfy the CPUID specification, but
>> did not properly check for extended and standard leaves and also
>> didn't account for the index subleaf.
>> Beside that this rule only applies to CPUID intercepts, which is not
>> the only user of the kvm_find_cpuid_entry() function.
>>
>> So fix this algorithm and move it into kvm_emulate_cpuid().
>> 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.
>>
>> @@ -4996,6 +4990,19 @@ void kvm_emulate_cpuid(struct kvm_vcpu *vcpu)
>>   	kvm_register_write(vcpu, VCPU_REGS_RCX, 0);
>>   	kvm_register_write(vcpu, VCPU_REGS_RDX, 0);
>>   	best = kvm_find_cpuid_entry(vcpu, function, index);
>> +
>> +	/* if no match is found, check whether we exceed the vCPU's limit
>> +	 * and return the content of the highest valid standard leaf instead.
>> +	 * This is to satisfy the CPUID specification.
>> +	 */
>> +	if (!best) {
>> +		best = kvm_find_cpuid_entry(vcpu, function&  0x80000000, 0);
> 
> "highest valid standard leaf" means the second argument should be zero, no?

Weird, but somehow true. I fixed this is in a another version (following).

Thanks for spotting this.

Andre.

> 
>> +		if (best&&  best->eax<  function)
>> +			best = kvm_find_cpuid_entry(vcpu, best->eax, index);
>> +		else
>> +			best = NULL;
>> +	}
>> +
>>   	if (best) {
>>   		kvm_register_write(vcpu, VCPU_REGS_RAX, best->eax);
>>   		kvm_register_write(vcpu, VCPU_REGS_RBX, best->ebx);
> 
> 

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany