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

[David Woodhouse <dwmw2@infradead.org>]

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On Fri, 2022-02-25 at 16:40 +0000, Sean Christopherson wrote:
> 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.

Right.  But at least they'd be *consistent*.

I was actually making that change anyway, for the benefit of VMs where
we are intentionally scaling to a known, different, TSC frequency —
which is currently completely hosed when all the vCPUs set it for
themselves because the TSC sync then fails.

> 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.

Hm, would the world be a better place if we knew that the delta between
the unrefined and refined TSC values was always within the tolerance of
tsc_tolerance_ppm for which we wouldn't bother scaling anyway?

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