[PATCH] virtio: notify virtqueue via host notifier when available

[Stefan Hajnoczi <stefanha@redhat.com>]

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.

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>
---
Felipe and Yongji: Only tested with "make check".  Please try
vhost-user-scsi/blk and let us know if it fixes the issue.

 include/hw/virtio/virtio-bus.h | 7 +++++++
 hw/virtio/virtio.c             | 4 +++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/hw/virtio/virtio-bus.h b/include/hw/virtio/virtio-bus.h
index 38c9399cd4..28ca51cb4c 100644
--- a/include/hw/virtio/virtio-bus.h
+++ b/include/hw/virtio/virtio-bus.h
@@ -139,6 +139,13 @@ static inline VirtIODevice *virtio_bus_get_device(VirtioBusState *bus)
 
 /* Return whether the proxy allows ioeventfd.  */
 bool virtio_bus_ioeventfd_enabled(VirtioBusState *bus);
+
+/* Return whether ioeventfd has been started. */
+static inline bool virtio_bus_ioeventfd_started(VirtioBusState *bus)
+{
+    return bus->ioeventfd_started;
+}
+
 /* Start the ioeventfd. */
 int virtio_bus_start_ioeventfd(VirtioBusState *bus);
 /* Stop the ioeventfd. */
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 527df03bfd..abdcec00cd 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -1567,6 +1567,8 @@ static void virtio_queue_notify_vq(VirtQueue *vq)
 
 void virtio_queue_notify(VirtIODevice *vdev, int n)
 {
+    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
+    VirtioBusState *vbus = VIRTIO_BUS(qbus);
     VirtQueue *vq = &vdev->vq[n];
 
     if (unlikely(!vq->vring.desc || vdev->broken)) {
@@ -1574,7 +1576,7 @@ void virtio_queue_notify(VirtIODevice *vdev, int n)
     }
 
     trace_virtio_queue_notify(vdev, vq - vdev->vq, vq);
-    if (vq->handle_aio_output) {
+    if (virtio_bus_ioeventfd_started(vbus)) {
         event_notifier_set(&vq->host_notifier);
     } else if (vq->handle_output) {
         vq->handle_output(vdev, vq);
-- 
2.21.0