When virtio_user is used with OVS-DPDK (with mq disabled), it cannot
receive any packets.

It's because when vhost provides VHOST_USER_GET_PROTOCOL_FEATURES,
all queue pairs are initialized in the disabled state. Quote
QEMU/docs/specs/vhost-user.txt:
    If VHOST_USER_F_PROTOCOL_FEATURES has not been negotiated, the
    ring is initialized in an enabled state.
    If VHOST_USER_F_PROTOCOL_FEATURES has been negotiated, the ring
    is initialized in a disabled state.

In OVS-DPDK, all queue pairs are in the disabled state by default.
When used with QEMU, QEMU will set it as enabled in the process of
initialization. So the fix here is to include similar logic in
virtio_user.

Fixes: 37a7eb2ae816 ("net/virtio-user: add device emulation layer")

Reported-by: Ning Li <redacted>
Signed-off-by: Jianfeng Tan <redacted>
---
 drivers/net/virtio/virtio_user/virtio_user_dev.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/virtio/virtio_user/virtio_user_dev.c b/drivers/net/virtio/virtio_user/virtio_user_dev.c
index 376c9cf..2c4e999 100644
--- a/drivers/net/virtio/virtio_user/virtio_user_dev.c
+++ b/drivers/net/virtio/virtio_user/virtio_user_dev.c

@@ -131,6 +131,13 @@ virtio_user_start_device(struct virtio_user_dev *dev)
 		}
 	}
 
+	/* As this feature is negotiated from the vhost, all queues are
+	 * initialized in the disabled state. For non-mq case, we enable
+	 * the 1st queue pair by default.
+	 */
+	if (dev->features & (1ull << VHOST_USER_GET_PROTOCOL_FEATURES))
+		vhost_user_enable_queue_pair(dev->vhostfd, 0, 1);
+
 	/* After setup all virtqueues, we need to set_features so that these
 	 * features can be set into each virtqueue in vhost side. And before
 	 * that, make sure VHOST_USER_F_PROTOCOL_FEATURES is added if mq is

-- 
2.7.4