Re: [PATCH v2 3/5] KVM: x86: Use common 'enable_apicv' variable for both APICv and AVIC

[Paolo Bonzini <pbonzini@redhat.com>]

On 24/05/21 18:52, Sean Christopherson wrote:
> On Mon, May 24, 2021, Paolo Bonzini wrote:
>> On 18/05/21 22:39, Sean Christopherson wrote:
>>>> +/* enable / disable AVIC */
>>>> +static int avic;
>>>> +module_param(avic, int, 0444);
>>> We should opportunistically make avic a "bool".
>>>
>>
>> And also:
>>
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index 11714c22c9f1..48cb498ff070 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -185,9 +185,12 @@ module_param(vls, int, 0444);
>>   static int vgif = true;
>>   module_param(vgif, int, 0444);
>> -/* enable / disable AVIC */
>> -static int avic;
>> -module_param(avic, int, 0444);
>> +/*
>> + * enable / disable AVIC.  Because the defaults differ for APICv
>> + * support between VMX and SVM we cannot use module_param_named.
>> + */
>> +static bool avic;
>> +module_param(avic, bool, 0444);
>>   bool __read_mostly dump_invalid_vmcb;
>>   module_param(dump_invalid_vmcb, bool, 0644);
>> @@ -1013,11 +1016,7 @@ static __init int svm_hardware_setup(void)
>>   			nrips = false;
>>   	}
>> -	if (!npt_enabled || !boot_cpu_has(X86_FEATURE_AVIC))
>> -		avic = false;
>> -
>> -	/* 'enable_apicv' is common between VMX/SVM but the defaults differ */
>> -	enable_apicv = avic;
>> +	enable_apicv = avic && npt_enabled && boot_cpu_has(X86_FEATURE_AVIC);
>>   	if (enable_apicv) {
>>   		pr_info("AVIC enabled\n");
>>
>> The "if" can come back when AVIC is enabled by default.
> 
> But "avic" is connected to the module param, even if it's off by default its
> effective value should be reflected in sysfs.  E.g. the user may incorrectly
> think AVIC is in use if they set avic=1 but the CPU doesn't support AVIC.
> Forcing the user to check /proc/cpuinfo or look for "AVIC enabled" in dmesg is
> kludgy at best.

Indeed -- I even tested the above, but only before realizing that 
module_param_named would change the default.  So for now this needs to
be "enable_apicv = avic = ...", and later it can become just

	enable_apicv &= npt_enabled && boot_cpu_has(X86_FEATURE_AVIC);

Paolo