Re: [PATCH v3 3/4] KVM: x86: Tie Intel and AMD behavior for MSR_TSC_AUX to guest CPU model

[Reiji Watanabe]

> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1610,6 +1610,29 @@ static int __kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data,
>                  * invokes 64-bit SYSENTER.
>                  */
>                 data = get_canonical(data, vcpu_virt_addr_bits(vcpu));
> +               break;
> +       case MSR_TSC_AUX:
> +               if (!kvm_cpu_cap_has(X86_FEATURE_RDTSCP))
> +                       return 1;
> +
> +               if (!host_initiated &&
> +                   !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> +                       return 1;
> +
> +               /*
> +                * Per Intel's SDM, bits 63:32 are reserved, but AMD's APM has
> +                * incomplete and conflicting architectural behavior.  Current
> +                * AMD CPUs completely ignore bits 63:32, i.e. they aren't
> +                * reserved and always read as zeros.  Enforce Intel's reserved
> +                * bits check if and only if the guest CPU is Intel, and clear
> +                * the bits in all other cases.  This ensures cross-vendor
> +                * migration will provide consistent behavior for the guest.
> +                */
> +               if (guest_cpuid_is_intel(vcpu) && (data >> 32) != 0)
> +                       return 1;
> +
> +               data = (u32)data;
> +               break;
>         }
>
>         msr.data = data;
> @@ -1646,6 +1669,17 @@ int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
>         if (!host_initiated && !kvm_msr_allowed(vcpu, index, KVM_MSR_FILTER_READ))
>                 return KVM_MSR_RET_FILTERED;
>
> +       switch (index) {
> +       case MSR_TSC_AUX:
> +               if (!kvm_cpu_cap_has(X86_FEATURE_RDTSCP))
> +                       return 1;
> +
> +               if (!host_initiated &&
> +                   !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> +                       return 1;


It looks Table 2-2 of the Intel SDM Vol4 (April 2021) says
TSC_AUX is supported:

   If CPUID.80000001H:EDX[27] = 1 or CPUID.(EAX=7,ECX=0):ECX[22] = 1

Should we also check X86_FEATURE_RDPID before returning 1
due to no RDTSCP support ?
There doesn't seem to be a similar description in the APM though.

Thanks,
Reiji