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

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Fri, 2026-07-24 at 14:19 -0700, Sean Christopherson wrote:
> 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?

I think I tried that first and it didn't work because
KVM_REQ_MASTERCLOCK_UPDATE wasn't actually set yet. I remember going
round in circles a few times here. I'll take another look.

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