linux-kernel - Re: [PATCH][v6] KVM: X86: support APERF/MPERF registers

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Date:   Fri, 5 Jun 2020 08:35:33 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Li RongQing <lirongqing@...du.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org,
        hpa@...or.com, bp@...en8.de, mingo@...hat.com, tglx@...utronix.de,
        jmattson@...gle.com, wanpengli@...cent.com, vkuznets@...hat.com,
        sean.j.christopherson@...el.com, wei.huang2@....com
Subject: Re: [PATCH][v6] KVM: X86: support APERF/MPERF registers

On 05/06/20 07:00, Xiaoyao Li wrote:
> you could do
> 
>     bool guest_cpuid_aperfmperf = false;
>     if (best)
>         guest_cpuid_aperfmperf = !!(best->ecx & BIT(0));
> 
>     if (guest_cpuid_aperfmerf != guest_has_aperfmperf(vcpu->kvm))
>         return -EINVAL;
> 
> 
> In fact, I think we can do nothing here. Leave it as what usersapce
> wants just like how KVM treats other CPUID bits.

The reason to do it like Rongqing did is that it's suggested to take the
output of KVM_GET_SUPPORTED_CPUID and pass it down to KVM_SET_CPUID2.
Unfortunately we have KVM_GET_SUPPORTED_CPUID as a /dev/kvm (not VM)
ioctl, otherwise you could have used guest_has_aperfmperf to affect the
output of KVM_GET_SUPPORTED_CPUID.

I think it's okay however to keep it simple as you suggest.  In this
case however __do_cpuid_func must not return the X86_FEATURE_APERFMPERF bit.

The guest can instead check for the availability of KVM_CAP_APERFMPERF,
which is already done in Rongqing's patch.

>> @@ -4930,6 +4939,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>>           kvm->arch.exception_payload_enabled = cap->args[0];
>>           r = 0;
>>           break;
>> +    case KVM_CAP_APERFMPERF:
>> +        kvm->arch.aperfmperf_mode =
>> +            boot_cpu_has(X86_FEATURE_APERFMPERF) ? cap->args[0] : 0;
> 
> Shouldn't check whether cap->args[0] is a valid value?

Yes, only valid values should be allowed.

Also, it should fail with -EINVAL if the host does not have
X86_FEATURE_APERFMPERF.

Thanks,

Paolo