Re: [PATCH 09/10] KVM: x86: Setup Hyper-V TSC page before Xen PV clocks (during clock update)

[Sean Christopherson <seanjc@google.com>]

On Mon, Jan 20, 2025, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
> > When updating paravirtual clocks, setup the Hyper-V TSC page before
> > Xen PV clocks.  This will allow dropping xen_pvclock_tsc_unstable in favor
> > of simply clearing PVCLOCK_TSC_STABLE_BIT in the reference flags.
> >
> > Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/kvm/x86.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 9eabd70891dd..c68e7f7ba69d 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -3280,6 +3280,8 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> >  		hv_clock.flags &= ~PVCLOCK_GUEST_STOPPED;
> >  	}
> >  
> > +	kvm_hv_setup_tsc_page(v->kvm, &hv_clock);
> > +
> >  #ifdef CONFIG_KVM_XEN
> >  	if (vcpu->xen.vcpu_info_cache.active)
> >  		kvm_setup_guest_pvclock(&hv_clock, v, &vcpu->xen.vcpu_info_cache,
> > @@ -3289,7 +3291,6 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> >  		kvm_setup_guest_pvclock(&hv_clock, v, &vcpu->xen.vcpu_time_info_cache, 0,
> >  					xen_pvclock_tsc_unstable);
> >  #endif
> > -	kvm_hv_setup_tsc_page(v->kvm, &hv_clock);
> >  	return 0;
> >  }
> 
> "No functional change detected".
> 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> 
> (What I'm wondering is if (from mostly theoretical PoV) it's OK to pass
> *some* of the PV clocks as stable and some as unstable to the same
> guest, i.e. if it would make sense to disable Hyper-V TSC page when
> KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE too.

I think it's ok to keep the Hyper-V TSC page in this case.  It's not that the Xen
PV clock is truly unstable, it's that some guests get tripped up by the STABLE
flag.  A guest that can't handle the STABLE flag has bigger problems than the
existence of a completely unrelated clock that is implied to be stable.

> I don't know if anyone combines Xen and Hyper-V emulation capabilities for
> the same guest on KVM though.)

That someone would have to be quite "brave" :-D