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    SubjectRe: [PATCH][v5] KVM: X86: support APERF/MPERF registers
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    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);

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