Re: [PATCH] KVM: x86: Don't snapshot "max" TSC if host TSC is constant

[Sean Christopherson <seanjc@google.com>]

On Fri, Feb 25, 2022, David Woodhouse wrote:
> On Fri, 2022-02-25 at 01:39 +0000, Sean Christopherson wrote:
> > @@ -11160,7 +11162,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> >         vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
> >         kvm_vcpu_mtrr_init(vcpu);
> >         vcpu_load(vcpu);
> > -       kvm_set_tsc_khz(vcpu, max_tsc_khz);
> > +       kvm_set_tsc_khz(vcpu, max_tsc_khz ? : tsc_khz);
> >         kvm_vcpu_reset(vcpu, false);
> >         kvm_init_mmu(vcpu);
> >         vcpu_put(vcpu);
> > 
> 
> Hm, now if you hit that race you end up potentially giving *different*
> frequencies to different vCPUs in a single guest, depending on when
> they were created.

Yep.  Though the race is much harder to hit (userspace vs TSC refinement).  The
existing race being hit is essentially do_initcalls() vs. TSC refinement.  

> How about this... (and as noted, I think I want to add an explicit KVM
> ioctl to set kvm->arch.default_tsc_khz for subsequently created vCPUs).

This wouldn't necessarily help.  E.g. assuming userspace knows the actual TSC
frequency, creating a vCPU before refinement completes might put the vCPU in
"always catchup" purgatory.

To really fix the race, KVM needs a notification that refinement completed (or
failed).  KVM could simply refuse to create vCPUs until it got the notification.
In the non-constant case, KVM would also need to refresh max_tsc_khz.