Re: [PATCH v7 31/36] KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for accurate KVM clock migration
Sean Christopherson <[email protected]>
| Newsgroups | org.xenproject.lists.xen-devel,org.kernel.vger.kvm,org.kernel.vger.linux-doc,org.kernel.vger.linux-kernel,org.kernel.vger.linux-kselftest |
|---|---|
| Message-ID | <[email protected]> |
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?