Re: [PATCH v7 31/36] KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for accurate KVM clock migration

[Sean Christopherson <seanjc@google.com>]

On Sat, Aug 01, 2026, David Woodhouse wrote:
> On Fri, 2026-07-31 at 16:24 -0700, Sean Christopherson wrote:
> > 
> > > +	/*
> > > +	 * Allow for a discrepancy of 1 kHz either way between the TSC
> > > +	 * frequency used to generate the user's pvclock and the current
> > > +	 * host's measured frequency, since they may not precisely match.
> > > +	 */
> > > +	if (user_tsc_hz < curr_tsc_hz - 1000 ||
> > > +	    user_tsc_hz > curr_tsc_hz + 1000) {
> > 
> > I don't follow, why is KVM restricting what frequency userspace can set?
> 
> Userspace actually sets the frequency with KVM_SET_TSC_KHZ. What KVM is
> insisting upon here is that the input to KVM_SET_CLOCK_GUEST is
> *consistent* with the guest's TSC frequency (within a little slop
> caused by different host TSCs).

Why does KVM care though?  I know some people hate that KVM's uAPI is permissive
to a fault, but trying to "help" userspace often ends badly for everyone.  E.g.
what happens if userspace does KVM_SET_TSC_KHZ after KVM_SET_CLOCK_GUEST?