hw/net/virtio-net: fix qemu set used ring flag even vhost started

When vhost-user or vhost-kernel is handling virtio net datapath,
QEMU should not touch used ring.

But with vhost-user socket reconnect scenario, in a very rare case
(has pending kick event). VRING_USED_F_NO_NOTIFY is set by QEMU in
following code path:

	#0  virtio_queue_split_set_notification (vq=0x7ff5f4c920a8, enable=0) at ../hw/virtio/virtio.c:511
	#1  0x0000559d6dbf033b in virtio_queue_set_notification (vq=0x7ff5f4c920a8, enable=0) at ../hw/virtio/virtio.c:576
	#2  0x0000559d6dbbbdbc in virtio_net_handle_tx_bh (vdev=0x559d703a6aa0, vq=0x7ff5f4c920a8) at ../hw/net/virtio-net.c:2801
	#3  0x0000559d6dbf4791 in virtio_queue_notify_vq (vq=0x7ff5f4c920a8) at ../hw/virtio/virtio.c:2248
	#4  0x0000559d6dbf79da in virtio_queue_host_notifier_read (n=0x7ff5f4c9211c) at ../hw/virtio/virtio.c:3525
	qemu#5  0x0000559d6d9a5814 in virtio_bus_cleanup_host_notifier (bus=0x559d703a6a20, n=1) at ../hw/virtio/virtio-bus.c:321
	qemu#6  0x0000559d6dbf83c9 in virtio_device_stop_ioeventfd_impl (vdev=0x559d703a6aa0) at ../hw/virtio/virtio.c:3774
	qemu#7  0x0000559d6d9a55c8 in virtio_bus_stop_ioeventfd (bus=0x559d703a6a20) at ../hw/virtio/virtio-bus.c:259
	qemu#8  0x0000559d6d9a53e8 in virtio_bus_grab_ioeventfd (bus=0x559d703a6a20) at ../hw/virtio/virtio-bus.c:199
	qemu#9  0x0000559d6dbf841c in virtio_device_grab_ioeventfd (vdev=0x559d703a6aa0) at ../hw/virtio/virtio.c:3783
	qemu#10 0x0000559d6d9bde18 in vhost_dev_enable_notifiers (hdev=0x559d707edd70, vdev=0x559d703a6aa0) at ../hw/virtio/vhost.c:1592
	qemu#11 0x0000559d6d89a0b8 in vhost_net_start_one (net=0x559d707edd70, dev=0x559d703a6aa0) at ../hw/net/vhost_net.c:266
	qemu#12 0x0000559d6d89a6df in vhost_net_start (dev=0x559d703a6aa0, ncs=0x559d7048d890, data_queue_pairs=31, cvq=0) at ../hw/net/vhost_net.c:412
	qemu#13 0x0000559d6dbb5b89 in virtio_net_vhost_status (n=0x559d703a6aa0, status=15 '\017') at ../hw/net/virtio-net.c:311
	qemu#14 0x0000559d6dbb5e34 in virtio_net_set_status (vdev=0x559d703a6aa0, status=15 '\017') at ../hw/net/virtio-net.c:392
	qemu#15 0x0000559d6dbb60d8 in virtio_net_set_link_status (nc=0x559d7048d890) at ../hw/net/virtio-net.c:455
	qemu#16 0x0000559d6da64863 in qmp_set_link (name=0x559d6f0b83d0 "hostnet1", up=true, errp=0x7ffdd76569f0) at ../net/net.c:1459
	qemu#17 0x0000559d6da7226e in net_vhost_user_event (opaque=0x559d6f0b83d0, event=CHR_EVENT_OPENED) at ../net/vhost-user.c:301
	qemu#18 0x0000559d6ddc7f63 in chr_be_event (s=0x559d6f2ffea0, event=CHR_EVENT_OPENED) at ../chardev/char.c:62
	qemu#19 0x0000559d6ddc7fdc in qemu_chr_be_event (s=0x559d6f2ffea0, event=CHR_EVENT_OPENED) at ../chardev/char.c:82

This issue causes guest kernel stop kicking device and traffic stop.

Add vhost_started check in virtio_net_handle_tx_bh to fix this wrong
VRING_USED_F_NO_NOTIFY set.

Signed-off-by: Yajun Wu <yajunw@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20240402045109.97729-1-yajunw@nvidia.com>
[PMD: Use unlikely()]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

hw/net/virtio-net.c

@@ -2865,6 +2865,10 @@ static void virtio_net_handle_tx_bh(VirtIODevice *vdev, VirtQueue *vq)
     VirtIONet *n = VIRTIO_NET(vdev);
     VirtIONetQueue *q = &n->vqs[vq2q(virtio_get_queue_index(vq))];
 
+    if (unlikely(n->vhost_started)) {
+        return;
+    }
+
     if (unlikely((n->status & VIRTIO_NET_S_LINK_UP) == 0)) {
         virtio_net_drop_tx_queue_data(vdev, vq);
         return;