virtio: notify virtqueue via host notifier when available

Host notifiers are used in several cases:
1. Traditional ioeventfd where virtqueue notifications are handled in
   the main loop thread.
2. IOThreads (aio_handle_output) where virtqueue notifications are
   handled in an IOThread AioContext.
3. vhost where virtqueue notifications are handled by kernel vhost or
   a vhost-user device backend.

Most virtqueue notifications from the guest use the ioeventfd mechanism,
but there are corner cases where QEMU code calls virtio_queue_notify().
This currently honors the host notifier for the IOThreads
aio_handle_output case, but not for the vhost case.  The result is that
vhost does not receive virtqueue notifications from QEMU when
virtio_queue_notify() is called.

This patch extends virtio_queue_notify() to set the host notifier
whenever it is enabled instead of calling the vq->(aio_)handle_output()
function directly.  We track the host notifier state for each virtqueue
separately since some devices may use it only for certain virtqueues.

This fixes the vhost case although it does add a trip through the
eventfd for the traditional ioeventfd case.  I don't think it's worth
adding a fast path for the traditional ioeventfd case because calling
virtio_queue_notify() is rare when ioeventfd is enabled.

Reported-by: Felipe Franciosi <felipe@nutanix.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20191105140946.165584-1-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-bus.c

@@ -288,6 +288,10 @@ int virtio_bus_set_host_notifier(VirtioBusState *bus, int n, bool assign)
         k->ioeventfd_assign(proxy, notifier, n, false);
     }
 
+    if (r == 0) {
+        virtio_queue_set_host_notifier_enabled(vq, assign);
+    }
+
     return r;
 }
 

hw/virtio/virtio.c

@@ -128,6 +128,7 @@ struct VirtQueue
     VirtIODevice *vdev;
     EventNotifier guest_notifier;
     EventNotifier host_notifier;
+    bool host_notifier_enabled;
     QLIST_ENTRY(VirtQueue) node;
 };
 
@@ -2271,7 +2272,7 @@ void virtio_queue_notify(VirtIODevice *vdev, int n)
     }
 
     trace_virtio_queue_notify(vdev, vq - vdev->vq, vq);
-    if (vq->handle_aio_output) {
+    if (vq->host_notifier_enabled) {
         event_notifier_set(&vq->host_notifier);
     } else if (vq->handle_output) {
         vq->handle_output(vdev, vq);
@@ -3145,6 +3146,7 @@ void virtio_init(VirtIODevice *vdev, const char *name,
         vdev->vq[i].vector = VIRTIO_NO_VECTOR;
         vdev->vq[i].vdev = vdev;
         vdev->vq[i].queue_index = i;
+        vdev->vq[i].host_notifier_enabled = false;
     }
 
     vdev->name = name;
@@ -3436,6 +3438,11 @@ EventNotifier *virtio_queue_get_host_notifier(VirtQueue *vq)
     return &vq->host_notifier;
 }
 
+void virtio_queue_set_host_notifier_enabled(VirtQueue *vq, bool enabled)
+{
+    vq->host_notifier_enabled = enabled;
+}
+
 int virtio_queue_set_host_notifier_mr(VirtIODevice *vdev, int n,
                                       MemoryRegion *mr, bool assign)
 {

include/hw/virtio/virtio.h

@@ -312,6 +312,7 @@ int virtio_device_grab_ioeventfd(VirtIODevice *vdev);
 void virtio_device_release_ioeventfd(VirtIODevice *vdev);
 bool virtio_device_ioeventfd_enabled(VirtIODevice *vdev);
 EventNotifier *virtio_queue_get_host_notifier(VirtQueue *vq);
+void virtio_queue_set_host_notifier_enabled(VirtQueue *vq, bool enabled);
 void virtio_queue_host_notifier_read(EventNotifier *n);
 void virtio_queue_aio_set_host_notifier_handler(VirtQueue *vq, AioContext *ctx,
                                                 VirtIOHandleAIOOutput handle_output);