Re: [PATCH v6 08/36] KVM: x86: Activate master clock immediately on vCPU creation

[Sean Christopherson <seanjc@xxxxxxxxxx>]

On Fri, Jul 03, 2026, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
> 
> Previously, the master clock was only activated when the first vCPU
> processed KVM_REQ_MASTERCLOCK_UPDATE during KVM_RUN. This meant that
> KVM_GET_CLOCK could not return the host_tsc field until after the
> first KVM_RUN, making it impossible for userspace to follow the
> documented TSC migration procedure without a dummy vCPU run.
> 
> Fix this by calling kvm_update_masterclock() directly from
> kvm_arch_vcpu_postcreate(), after kvm_synchronize_tsc() has already
> set all_vcpus_matched_freq. This ensures the master clock is active
> immediately, and KVM_GET_CLOCK returns a valid {host_tsc, realtime}
> pair as soon as a vCPU exists.
> 
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ff45577ed90c..2039bd8518fb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -13110,6 +13110,8 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
>               return;
>       vcpu_load(vcpu);
>       kvm_synchronize_tsc(vcpu, NULL);
> +     if (!vcpu->kvm->arch.use_master_clock)

Any reason this can't be?

        if (kvm_check_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu))
                kvm_update_masterclock(vcpu->kvm);

> +             kvm_update_masterclock(vcpu->kvm);

I don't love doing work outside of KVM_RUN that is typically handled by KVM_RUN,
but this seems fine?

>       vcpu_put(vcpu);
>  
>       /* poll control enabled by default */
> -- 
> 2.54.0
>