KVM: x86: zero kvmclock_offset when vcpu0 initializes kvmclock system MSR

Initialize kvmclock base, on kvmclock system MSR write time,
so that the guest sees kvmclock counting from zero.

This matches baremetal behaviour when kvmclock in guest
sets sched clock stable.
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>
[Remove unnecessary comment. - Paolo]
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -1700,6 +1700,8 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 		vcpu->pvclock_set_guest_stopped_request = false;
 	}
 
+	pvclock_flags |= PVCLOCK_COUNTS_FROM_ZERO;
+
 	/* If the host uses TSC clocksource, then it is stable */
 	if (use_master_clock)
 		pvclock_flags |= PVCLOCK_TSC_STABLE_BIT;
@@ -2282,6 +2284,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 					&vcpu->requests);
 
 			ka->boot_vcpu_runs_old_kvmclock = tmp;
+
+			ka->kvmclock_offset = -get_kernel_ns();
 		}
 
 		vcpu->arch.time = data;