KVM: x86: fix maintaining of kvm_clock stability on guest CPU hotplug

VCPU TSC synchronization is perfromed in kvm_write_tsc() when the TSC
value being set is within 1 second from the expected, as obtained by
extrapolating of the TSC in already synchronized VCPUs.

This is naturally achieved on all VCPUs at VM start and resume;
however on VCPU hotplug it is not: the newly added VCPU is created
with TSC == 0 while others are well ahead.

To compensate for that, consider host-initiated kvm_write_tsc() with
TSC == 0 a special case requiring synchronization regardless of the
current TSC on other VCPUs.
Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/x86.c

@@ -1454,6 +1454,14 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
 	elapsed = ns - kvm->arch.last_tsc_nsec;
 
 	if (vcpu->arch.virtual_tsc_khz) {
+		if (data == 0 && msr->host_initiated) {
+			/*
+			 * detection of vcpu initialization -- need to sync
+			 * with other vCPUs. This particularly helps to keep
+			 * kvm_clock stable after CPU hotplug
+			 */
+			synchronizing = true;
+		} else {
 			u64 tsc_exp = kvm->arch.last_tsc_write +
 						nsec_to_cycles(vcpu, elapsed);
 			u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;
@@ -1465,6 +1473,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
 			synchronizing = data < tsc_exp + tsc_hz &&
 					data + tsc_hz > tsc_exp;
 		}
+	}
 
 	/*
 	 * For a reliable TSC, we can match TSC offsets, and for an unstable