Re: [PATCH 2/3] virtio-net: Only enable userland vq if using tap backend

[Jason Wang]

On Thu, Nov 18, 2021 at 3:29 AM Eugenio Pérez <eperezma@redhat.com> wrote:
>
> Qemu falls back on userland handlers even if vhost-user and vhost-vdpa
> cases. These assumes a tap device can handle the packets.
>
> If a vdpa device fail to start, it can trigger a sigsegv because of
> that. Do not resort on them unless actually possible.

It would be better to show the calltrace here then we can see the root cause.

>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  include/hw/virtio/virtio.h |  2 ++
>  hw/net/virtio-net.c        |  4 ++++
>  hw/virtio/virtio.c         | 21 +++++++++++++--------
>  3 files changed, 19 insertions(+), 8 deletions(-)
>
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index 8bab9cfb75..1712ba0b4c 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -105,6 +105,8 @@ struct VirtIODevice
>      VMChangeStateEntry *vmstate;
>      char *bus_name;
>      uint8_t device_endian;
> +    /* backend does not support userspace handler */
> +    bool disable_ioeventfd_handler;
>      bool use_guest_notifier_mask;
>      AddressSpace *dma_as;
>      QLIST_HEAD(, VirtQueue) *vector_queues;
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 004acf858f..8c5c4e5a9d 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -3501,6 +3501,10 @@ static void virtio_net_device_realize(DeviceState 
> *dev, Error **errp)
>      nc = qemu_get_queue(n->nic);
>      nc->rxfilter_notify_enabled = 1;
>
> +    if (!nc->peer || nc->peer->info->type != NET_CLIENT_DRIVER_TAP) {
> +        /* Only tap can use userspace networking */
> +        vdev->disable_ioeventfd_handler = true;
> +    }
>      if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
>          struct virtio_net_config netcfg = {};
>          memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index ea7c079fb0..1e04db6650 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -3734,17 +3734,22 @@ static int 
> virtio_device_start_ioeventfd_impl(VirtIODevice *vdev)
>              err = r;
>              goto assign_error;
>          }
> -        event_notifier_set_handler(&vq->host_notifier,
> -                                   virtio_queue_host_notifier_read);
> +
> +        if (!vdev->disable_ioeventfd_handler) {
> +            event_notifier_set_handler(&vq->host_notifier,
> +                                       virtio_queue_host_notifier_read);

This is just about not responding to ioeventfd. Does this happen only
when ioeventfd is enabled? If yes, we probably need a consistent way
to deal with that. Will having a dummy receiver be more simpler?

Thanks

> +        }
>      }
>
> -    for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
> -        /* Kick right away to begin processing requests already in vring */
> -        VirtQueue *vq = &vdev->vq[n];
> -        if (!vq->vring.num) {
> -            continue;
> +    if (!vdev->disable_ioeventfd_handler) {
> +        for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
> +            /* Kick right away to begin processing requests already in vring 
> */
> +            VirtQueue *vq = &vdev->vq[n];
> +            if (!vq->vring.num) {
> +                continue;
> +            }
> +            event_notifier_set(&vq->host_notifier);
>          }
> -        event_notifier_set(&vq->host_notifier);
>      }
>      memory_region_transaction_commit();
>      return 0;
> --
> 2.27.0
>