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Subject: Re: [PATCH][v6] KVM: X86: support APERF/MPERF registers
To:     Xiaoyao Li <xiaoyao.li@intel.com>,
        Li RongQing <lirongqing@baidu.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org,
        hpa@zytor.com, bp@alien8.de, mingo@redhat.com, tglx@linutronix.de,
        jmattson@google.com, wanpengli@tencent.com, vkuznets@redhat.com,
        sean.j.christopherson@intel.com, wei.huang2@amd.com
References: <1591321466-2046-1-git-send-email-lirongqing@baidu.com>
 <b70d03dd-947f-dee5-5499-3b381372497d@intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <72a75924-c3cb-6b23-62bd-67b739dec166@redhat.com>
Date:   Fri, 5 Jun 2020 08:35:33 +0200
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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