Re: [PATCH v5 06/34] KVM: x86: Explicitly disable TSC scaling without CONSTANT_TSC

[Sean Christopherson <seanjc@google.com>]

On Mon, Jun 08, 2026, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> KVM does make an attempt to cope with non-constant TSC, and has
> notifiers to handle host TSC frequency changes. However, it *only*
> adjusts the KVM clock, and doesn't adjust TSC frequency scaling when
> the host changes.
> 
> This is presumably because non-constant TSCs were fixed in hardware
> long before TSC scaling was implemented, so there should never be real
> CPUs which have TSC scaling but *not* CONSTANT_TSC.
> 
> Such a combination could potentially happen in some odd L1 nesting
> environment, but it isn't worth trying to support it. Just make the
> dependency explicit.
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Reviewed-by: Paul Durrant <paul@xen.org>
> ---
>  arch/x86/kvm/svm/svm.c | 3 ++-
>  arch/x86/kvm/vmx/vmx.c | 2 +-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index e7fdd7a9c280..7817752533fe 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5546,7 +5546,8 @@ static __init int svm_hardware_setup(void)
>  				     XFEATURE_MASK_BNDCSR);
>  
>  	if (tsc_scaling) {
> -		if (!boot_cpu_has(X86_FEATURE_TSCRATEMSR)) {
> +		if (!boot_cpu_has(X86_FEATURE_TSCRATEMSR) ||
> +		    !boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
>  			tsc_scaling = false;
>  		} else {
>  			pr_info("TSC scaling supported\n");
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index a29896a9ef14..ed207cc7692d 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -8672,7 +8672,7 @@ __init int vmx_hardware_setup(void)
>  	if (!enable_apicv || !cpu_has_vmx_ipiv())
>  		enable_ipiv = false;
>  
> -	if (cpu_has_vmx_tsc_scaling())
> +	if (cpu_has_vmx_tsc_scaling() && boot_cpu_has(X86_FEATURE_CONSTANT_TSC))

We should clear SECONDARY_EXEC_TSC_SCALING in setup_vmcs_config().  Failure to
clear the vmcs_config bit will advertise the feature to L2 and allow it to be
enabled in vmcs12, but KVM will ultimately not honor the scaling in vmcs02:

	if (kvm_caps.has_tsc_control)
		vmcs_write64(TSC_MULTIPLIER, vcpu->arch.tsc_scaling_ratio);

This series kinda sorta gets there with "KVM: x86: Remove pvclock_gtod_data and
private timekeeping code", but that change looks misplaced?  And I think clearing
the bit this late will lead to false failures in vmx_check_processor_compat() due
to the golden config clearing the bit, but the local config having it set.


>  		kvm_caps.has_tsc_control = true;
>  
>  	kvm_caps.max_tsc_scaling_ratio = KVM_VMX_TSC_MULTIPLIER_MAX;
> -- 
> 2.54.0
>