Re: [PATCH 0/2] RFC: Precise TSC migration

[Paolo Bonzini]

On 30/11/20 17:54, Andy Lutomirski wrote:
> > I*do think*  however that we should redefine KVM_CLOCK_TSC_STABLE
> > in the documentation to state that it only guarantees invariance if the guest
> > doesn't mess with its own TSC.
> >
> > Also I think we should consider enabling the X86_FEATURE_TSC_RELIABLE
> > in the guest kernel, when kvm is detected to avoid the guest even from trying
> > to sync TSC on newly hotplugged vCPUs.
> >
> > (The guest doesn't end up touching TSC_ADJUST usually, but it still might
> > in some cases due to scheduling of guest vCPUs)
> >
> > (X86_FEATURE_TSC_RELIABLE short circuits tsc synchronization on CPU hotplug,
> > and TSC clocksource watchdog, and the later we might want to keep).
> If you're going to change the guest behavior to be more trusting of
> the host, I think
> the host should probably signal this to the guest using a new bit.

Yes, a new CPUID bit takes longer to propagate to existing setups, but 
it is more future proof.

Paolo