[Qemu-devel] [RFC 08/14] vhost: make sure call fd has been received

[marcandre.lureau@redhat.com]

From: Marc-André Lureau <marcandre.lureau@redhat.com>

vhost switches between masked notifier and guest notifier when
unmasking. virtio_pci_vq_vector_unmask() checks if there was pending
notification, but at the time it is checked, vhost-user backend doesn't
guarantee that the switch happened yet, so it may lose some events.

To solve this vhost-user race, I introduced an extra "sync"
call (waiting for a reply). One may want to make all/many vhost-user
replies mandatories by adding a new capability instead.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 hw/virtio/vhost.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 4400718..692c38f 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -1181,6 +1181,7 @@ void vhost_virtqueue_mask(struct vhost_dev *hdev, VirtIODevice *vdev, int n,
     struct VirtQueue *vvq = virtio_get_queue(vdev, n);
     int r, index = n - hdev->vq_index;
     struct vhost_vring_file file;
+    uint64_t features;
 
     if (mask) {
         assert(vdev->use_guest_notifier_mask);
@@ -1192,6 +1193,10 @@ void vhost_virtqueue_mask(struct vhost_dev *hdev, VirtIODevice *vdev, int n,
     file.index = hdev->vhost_ops->vhost_get_vq_index(hdev, n);
     r = hdev->vhost_ops->vhost_set_vring_call(hdev, &file);
     assert(r >= 0);
+
+    /* silly sync call to make sure the call fd has been received */
+    r = hdev->vhost_ops->vhost_get_features(hdev, &features);
+    assert(r >= 0);
 }
 
 uint64_t vhost_get_features(struct vhost_dev *hdev, const int *feature_bits,
-- 
2.7.4