Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

[Paolo Bonzini]

On 08/12/20 18:08, Maxim Levitsky wrote:
> > Even if you support TSCADJUST and let the guest write to it does not
> > change the per guest offset at all. TSCADJUST is per [v]CPU and adds on
> > top:
> >
> >      tscvcpu = tsc_host + guest_offset + TSC_ADJUST
> >
> > Scaling is just orthogonal and does not change any of this.
>
> I agree with this, and I think that this is what we will end up doing.
> Paulo, what do you think about this?

Yes, you can have a VM ioctl that saves/restores cur_tsc_nsec and 
cur_tsc_write.  The restore side would loop on all vcpus.

However, it is not so easy: 1) it would have to be usable only if 
KVM_X86_QUIRK_TSC_HOST_ACCESS is false, 2) it would fail if 
kvm->arch.nr_vcpus_matched_tsc == kvm->online_vcpus (which basically 
means that userspace didn't mess up the TSC configuration).  If not, it 
would return -EINVAL.

Also, while at it let's burn and pour salt on the support for 
KVM_SET_TSC_KHZ unless TSC scaling is supported, together with 
vcpu->tsc_catchup and all the "tolerance" crap that is in 
kvm_set_tsc_khz.  And initialize vcpu->arch.virtual_tsc_khz to 
kvm->arch.last_tsc_khz before calling kvm_synchronize_tsc.

Paolo