Re: [PATCH][v5] KVM: X86: support APERF/MPERF registers

[Xiaoyao Li]

On 5/30/2020 12:35 PM, Li RongQing wrote:
> Guest kernel reports a fixed cpu frequency in /proc/cpuinfo,
> this is confused to user when turbo is enable, and aperf/mperf
> can be used to show current cpu frequency after 7d5905dc14a
> "(x86 / CPU: Always show current CPU frequency in /proc/cpuinfo)"
> so guest should support aperf/mperf capability
>
> This patch implements aperf/mperf by three mode: none, software
> emulation, and pass-through
>
> None: default mode, guest does not support aperf/mperf
>
> Software emulation: the period of aperf/mperf in guest mode are
> accumulated as emulated value
>
> Pass-though: it is only suitable for KVM_HINTS_REALTIME, Because
> that hint guarantees we have a 1:1 vCPU:CPU binding and guaranteed
> no over-commit.
>
> And a per-VM capability is added to configure aperfmperf mode

[...]

> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index cd708b0b460a..c960dda4251b 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -122,6 +122,14 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
>   					   MSR_IA32_MISC_ENABLE_MWAIT);
>   	}
>   
> +	best = kvm_find_cpuid_entry(vcpu, 6, 0);
> +	if (best) {
> +		if (guest_has_aperfmperf(vcpu->kvm) &&
> +			boot_cpu_has(X86_FEATURE_APERFMPERF))
> +			best->ecx |= 1;
> +		else
> +			best->ecx &= ~1;
> +	}

In my understanding, KVM allows userspace to set a CPUID feature bit for 
guest even if hardware doesn't support the feature.

So what makes X86_FEATURE_APERFMPERF different here? Is there any 
concern I miss?

-Xiaoyao

>   	/* Note, maxphyaddr must be updated before tdp_level. */
>   	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
>   	vcpu->arch.tdp_level = kvm_x86_ops.get_tdp_level(vcpu);