Re: [PATCH] virtio-net: fix qemu set used ring flag even vhost started

[Philippe Mathieu-Daudé]

Hi Yajun,

On 2/4/24 06:51, Yajun Wu wrote:
> When vhost-user or vhost-kernel is handling virtio net datapath, qemu

"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

"QEMU"

> 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
>         #5  0x0000559d6d9a5814 in virtio_bus_cleanup_host_notifier 
> (bus=0x559d703a6a20, n=1) at ../hw/virtio/virtio-bus.c:321
>         #6  0x0000559d6dbf83c9 in virtio_device_stop_ioeventfd_impl 
> (vdev=0x559d703a6aa0) at ../hw/virtio/virtio.c:3774
>         #7  0x0000559d6d9a55c8 in virtio_bus_stop_ioeventfd 
> (bus=0x559d703a6a20) at ../hw/virtio/virtio-bus.c:259
>         #8  0x0000559d6d9a53e8 in virtio_bus_grab_ioeventfd 
> (bus=0x559d703a6a20) at ../hw/virtio/virtio-bus.c:199
>         #9  0x0000559d6dbf841c in virtio_device_grab_ioeventfd 
> (vdev=0x559d703a6aa0) at ../hw/virtio/virtio.c:3783
>         #10 0x0000559d6d9bde18 in vhost_dev_enable_notifiers 
> (hdev=0x559d707edd70, vdev=0x559d703a6aa0) at ../hw/virtio/vhost.c:1592
>         #11 0x0000559d6d89a0b8 in vhost_net_start_one (net=0x559d707edd70, 
> dev=0x559d703a6aa0) at ../hw/net/vhost_net.c:266
>         #12 0x0000559d6d89a6df in vhost_net_start (dev=0x559d703a6aa0, 
> ncs=0x559d7048d890, data_queue_pairs=31, cvq=0) at ../hw/net/vhost_net.c:412
>         #13 0x0000559d6dbb5b89 in virtio_net_vhost_status (n=0x559d703a6aa0, 
> status=15 '\017') at ../hw/net/virtio-net.c:311
>         #14 0x0000559d6dbb5e34 in virtio_net_set_status (vdev=0x559d703a6aa0, 
> status=15 '\017') at ../hw/net/virtio-net.c:392
>         #15 0x0000559d6dbb60d8 in virtio_net_set_link_status 
> (nc=0x559d7048d890) at ../hw/net/virtio-net.c:455
>         #16 0x0000559d6da64863 in qmp_set_link (name=0x559d6f0b83d0 "hostnet1", 
> up=true, errp=0x7ffdd76569f0) at ../net/net.c:1459
>         #17 0x0000559d6da7226e in net_vhost_user_event (opaque=0x559d6f0b83d0, 
> event=CHR_EVENT_OPENED) at ../net/vhost-user.c:301
>         #18 0x0000559d6ddc7f63 in chr_be_event (s=0x559d6f2ffea0, 
> event=CHR_EVENT_OPENED) at ../chardev/char.c:62
>         #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>
> ---
>   hw/net/virtio-net.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index a6ff000cd9..8035e01fdf 100644
> --- a/hw/net/virtio-net.c
> +++ b/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 (n->vhost_started) {

Since you mentioned "in a very rare case", maybe use unlikely()?

> +        return;
> +    }
> +
>       if (unlikely((n->status & VIRTIO_NET_S_LINK_UP) == 0)) {
>           virtio_net_drop_tx_queue_data(vdev, vq);
>           return;