Re: [PATCH] KVM: hyperv: support HV_X64_MSR_TSC_FREQUENCY and HV_X64_MSR_APIC_FREQUENCY

["Radim Krčmář" <rkrcmar@redhat.com>]

I missed this patch last week, sorry.

2017-07-17 15:28+0200, Ladi Prosek:
> It has been experimentally confirmed that supporting these two MSRs is one
> of the necessary conditions for nested Hyper-V to use the TSC page. Modern
> Windows guests are noticeably slower when they fall back to reading
> timestamps from the HV_X64_MSR_TIME_REF_COUNT MSR instead of using the TSC
> page.
> 
> The newly supported MSRs are advertised with the AccessFrequencyRegs
> partition privilege flag and CPUID.40000003H:EDX[8] "Support for
> determining timer frequencies is available" (both outside of the scope of
> this KVM patch).

Do Windows ignore the AccessFrequencyMsrs (CPUID.40000003H:EAX[11]) bit?

> Hypervisor Top Level Functional Specification version 5.0b does not
> explicitly mark these MSRs as partition wide. This is, however, implied by
> the way Windows uses them (one read on Hyper-V startup regardless of the
> number of VCPUs) as well as our implementation.
> 
> Signed-off-by: Ladi Prosek <lprosek@redhat.com>
> ---
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> @@ -1065,6 +1067,12 @@ static int kvm_hv_get_msr_pw(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
>  	case HV_X64_MSR_REFERENCE_TSC:
>  		data = hv->hv_tsc_page;
>  		break;
> +	case HV_X64_MSR_TSC_FREQUENCY:
> +		data = (u64)tsc_khz * 1000;
> +		break;

This should be based on vcpu->arch.virtual_tsc_khz, because KVM can use
guest TSC scaling,

thanks.

> +	case HV_X64_MSR_APIC_FREQUENCY:
> +		data = APIC_BUS_FREQUENCY;
> +		break;
>  	case HV_X64_MSR_CRASH_P0 ... HV_X64_MSR_CRASH_P4:
>  		return kvm_hv_msr_get_crash_data(vcpu,
>  						 msr - HV_X64_MSR_CRASH_P0,