Re: [PATCH v9 13/20] virtio-net: Return an error when vhost cannot enable RSS

[Yuri Benditovich]

On Wed, Apr 3, 2024 at 2:11 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>
> vhost requires eBPF for RSS. When eBPF is not available, virtio-net
> implicitly disables RSS even if the user explicitly requests it. Return
> an error instead of implicitly disabling RSS if RSS is requested but not
> available.
>
> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> ---
>  hw/net/virtio-net.c | 97 
> ++++++++++++++++++++++++++---------------------------
>  1 file changed, 48 insertions(+), 49 deletions(-)
>
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 61b49e335dea..3d53eba88cfc 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -793,9 +793,6 @@ static uint64_t virtio_net_get_features(VirtIODevice 
> *vdev, uint64_t features,
>          return features;
>      }
>
> -    if (!ebpf_rss_is_loaded(&n->ebpf_rss)) {
> -        virtio_clear_feature(&features, VIRTIO_NET_F_RSS);
> -    }
>      features = vhost_net_get_features(get_vhost_net(nc->peer), features);
>      vdev->backend_features = features;
>
> @@ -3591,6 +3588,50 @@ static bool 
> failover_hide_primary_device(DeviceListener *listener,
>      return qatomic_read(&n->failover_primary_hidden);
>  }
>
> +static void virtio_net_device_unrealize(DeviceState *dev)
> +{
> +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> +    VirtIONet *n = VIRTIO_NET(dev);
> +    int i, max_queue_pairs;
> +
> +    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS)) {
> +        virtio_net_unload_ebpf(n);
> +    }
> +
> +    /* This will stop vhost backend if appropriate. */
> +    virtio_net_set_status(vdev, 0);
> +
> +    g_free(n->netclient_name);
> +    n->netclient_name = NULL;
> +    g_free(n->netclient_type);
> +    n->netclient_type = NULL;
> +
> +    g_free(n->mac_table.macs);
> +    g_free(n->vlans);
> +
> +    if (n->failover) {
> +        qobject_unref(n->primary_opts);
> +        device_listener_unregister(&n->primary_listener);
> +        migration_remove_notifier(&n->migration_state);
> +    } else {
> +        assert(n->primary_opts == NULL);
> +    }
> +
> +    max_queue_pairs = n->multiqueue ? n->max_queue_pairs : 1;
> +    for (i = 0; i < max_queue_pairs; i++) {
> +        virtio_net_del_queue(n, i);
> +    }
> +    /* delete also control vq */
> +    virtio_del_queue(vdev, max_queue_pairs * 2);
> +    qemu_announce_timer_del(&n->announce_timer, false);
> +    g_free(n->vqs);
> +    qemu_del_nic(n->nic);
> +    virtio_net_rsc_cleanup(n);
> +    g_free(n->rss_data.indirections_table);
> +    net_rx_pkt_uninit(n->rx_pkt);
> +    virtio_cleanup(vdev);
> +}
> +
>  static void virtio_net_device_realize(DeviceState *dev, Error **errp)
>  {
>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> @@ -3760,53 +3801,11 @@ static void virtio_net_device_realize(DeviceState 
> *dev, Error **errp)
>
>      net_rx_pkt_init(&n->rx_pkt);
>
> -    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS)) {
> -        virtio_net_load_ebpf(n);
> -    }
> -}
> -
> -static void virtio_net_device_unrealize(DeviceState *dev)
> -{
> -    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> -    VirtIONet *n = VIRTIO_NET(dev);
> -    int i, max_queue_pairs;
> -
> -    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS)) {
> -        virtio_net_unload_ebpf(n);
> +    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS) &&
> +        !virtio_net_load_ebpf(n) && get_vhost_net(nc->peer)) {
> +        virtio_net_device_unrealize(dev);
> +        error_setg(errp, "Can't load eBPF RSS for vhost");
>      }

As I already mentioned, I think this is an extremely bad idea to
fail to run qemu due to such a reason as .absence of one feature.
What I suggest is:
1. Redefine rss as tri-state (off|auto|on)
2. Fail to run only if rss is on and not available via ebpf
3. On auto - silently drop it
4. The same with 'hash' option - it is not compatible with vhost (at
least at the moment)
5. Reformat the patch as it is hard to review it due to replacing
entire procedures, i.e. one patch with replacing without changes,
another one - with real changes.
If this is hard to review only for me - please ignore that.

> -
> -    /* This will stop vhost backend if appropriate. */
> -    virtio_net_set_status(vdev, 0);
> -
> -    g_free(n->netclient_name);
> -    n->netclient_name = NULL;
> -    g_free(n->netclient_type);
> -    n->netclient_type = NULL;
> -
> -    g_free(n->mac_table.macs);
> -    g_free(n->vlans);
> -
> -    if (n->failover) {
> -        qobject_unref(n->primary_opts);
> -        device_listener_unregister(&n->primary_listener);
> -        migration_remove_notifier(&n->migration_state);
> -    } else {
> -        assert(n->primary_opts == NULL);
> -    }
> -
> -    max_queue_pairs = n->multiqueue ? n->max_queue_pairs : 1;
> -    for (i = 0; i < max_queue_pairs; i++) {
> -        virtio_net_del_queue(n, i);
> -    }
> -    /* delete also control vq */
> -    virtio_del_queue(vdev, max_queue_pairs * 2);
> -    qemu_announce_timer_del(&n->announce_timer, false);
> -    g_free(n->vqs);
> -    qemu_del_nic(n->nic);
> -    virtio_net_rsc_cleanup(n);
> -    g_free(n->rss_data.indirections_table);
> -    net_rx_pkt_uninit(n->rx_pkt);
> -    virtio_cleanup(vdev);
>  }
>
>  static void virtio_net_reset(VirtIODevice *vdev)
>
> --
> 2.44.0
>