virtio-pci: Disable virtio-ioeventfd when !CONFIG_IOTHREAD

It is not possible to use virtio-ioeventfd when building without an I/O
thread.  We rely on a signal to kick us out of vcpu execution.  Timers
and AIO use SIGALRM and SIGUSR2 respectively.  Unfortunately eventfd
does not support O_ASYNC (SIGIO) so eventfd cannot be used in a signal
driven manner.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

kvm-all.c

@@ -449,10 +449,14 @@ int kvm_check_extension(KVMState *s, unsigned int extension)
 
 static int kvm_check_many_ioeventfds(void)
 {
-    /* Older kernels have a 6 device limit on the KVM io bus.  Find out so we
+    /* Userspace can use ioeventfd for io notification.  This requires a host
+     * that supports eventfd(2) and an I/O thread; since eventfd does not
+     * support SIGIO it cannot interrupt the vcpu.
+     *
+     * Older kernels have a 6 device limit on the KVM io bus.  Find out so we
      * can avoid creating too many ioeventfds.
      */
-#ifdef CONFIG_EVENTFD
+#if defined(CONFIG_EVENTFD) && defined(CONFIG_IOTHREAD)
     int ioeventfds[7];
     int i, ret = 0;
     for (i = 0; i < ARRAY_SIZE(ioeventfds); i++) {