KVM: X86: trigger kvmclock sync request just once on VM creation

In the progress of vCPUs creation, it queues a kvmclock sync worker to the global
workqueue before each vCPU creation completes. The workqueue subsystem guarantees
not to queue the already queued work; however, we can make the logic more clear by
making just one leader to trigger this kvmclock sync request, and also save on
cacheline bouncing caused by test_and_set_bit.
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -9335,9 +9335,7 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
 
 	mutex_unlock(&vcpu->mutex);
 
-	if (!kvmclock_periodic_sync)
-		return;
-
+	if (kvmclock_periodic_sync && vcpu->vcpu_idx == 0)
 		schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
 						KVMCLOCK_SYNC_PERIOD);
 }