Re: [PATCH 3/3] virtio-net: graceful fallback to vhost=off for tap netdev

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 04, 2021 at 10:29:15PM +0200, Yuri Benditovich wrote:
> Currently virtio-net silently clears features if they are
> not supported by respective vhost. This may create migration
> problems in future if vhost features on the source and destination
> are different. Implement graceful fallback to no-vhost mode
> when some acked features contradict with vhost. The decision is
> taken on set_features call and the vhost will be disabled
> till next reset (or migration).
> Such fallback is currently enabled only for TAP netdev.
> 
> Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.com>


Sounds good, but I don't think we should do this if
vhostforce=on is set.

Also, let's document this behaviour with the vhost option so people
are not suprized.

> ---
>  hw/net/virtio-net.c | 58 ++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 50 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 5150f295e8..b353060e63 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -515,6 +515,15 @@ static RxFilterInfo *virtio_net_query_rxfilter(NetClientState *nc)
>      return info;
>  }
>  
> +static void virtio_net_allow_vhost(VirtIONet *n, bool allow)
> +{
> +    int i;
> +    for (i = 0; i < n->max_queues; i++) {
> +        NetClientState *nc = qemu_get_subqueue(n->nic, i)->peer;
> +        nc->vhost_net_disabled = !allow;
> +    }
> +}
> +
>  static void virtio_net_reset(VirtIODevice *vdev)
>  {
>      VirtIONet *n = VIRTIO_NET(vdev);
> @@ -552,6 +561,7 @@ static void virtio_net_reset(VirtIODevice *vdev)
>              assert(!virtio_net_get_subqueue(nc)->async_tx.elem);
>          }
>      }
> +    virtio_net_allow_vhost(n, true);
>  }
>  
>  static void peer_test_vnet_hdr(VirtIONet *n)
> @@ -689,6 +699,15 @@ static void virtio_net_set_queues(VirtIONet *n)
>      }
>  }
>  
> +static bool can_disable_vhost(VirtIONet *n)
> +{
> +    NetClientState *peer = qemu_get_queue(n->nic)->peer;
> +    if (!get_vhost_net(peer)) {
> +        return false;
> +    }
> +    return !peer || peer->info->type == NET_CLIENT_DRIVER_TAP;
> +}
> +
>  static void virtio_net_set_multiqueue(VirtIONet *n, int multiqueue);
>  
>  static uint64_t virtio_net_get_features(VirtIODevice *vdev, uint64_t features,
> @@ -725,14 +744,14 @@ static uint64_t virtio_net_get_features(VirtIODevice *vdev, uint64_t features,
>          return features;
>      }
>  
> -    virtio_clear_feature(&features, VIRTIO_NET_F_RSS);
> -    virtio_clear_feature(&features, VIRTIO_NET_F_HASH_REPORT);
> -    features = vhost_net_get_features(get_vhost_net(nc->peer), features);
> -    vdev->backend_features = features;
> +    vdev->backend_features = vhost_net_get_features(get_vhost_net(nc->peer), features);
>  
> -    if (n->mtu_bypass_backend &&
> -            (n->host_features & 1ULL << VIRTIO_NET_F_MTU)) {
> -        features |= (1ULL << VIRTIO_NET_F_MTU);
> +    if (!can_disable_vhost(n)) {
> +        features = vdev->backend_features;
> +        if (n->mtu_bypass_backend &&
> +                (n->host_features & 1ULL << VIRTIO_NET_F_MTU)) {
> +            features |= (1ULL << VIRTIO_NET_F_MTU);
> +        }
>      }
>  
>      return features;
> @@ -872,10 +891,25 @@ static void failover_add_primary(VirtIONet *n, Error **errp)
>      error_propagate(errp, err);
>  }
>  
> +static bool check_vhost_features(VirtIONet *n, uint64_t features)
> +{
> +    NetClientState *nc = qemu_get_queue(n->nic);
> +    uint64_t filtered;
> +    if (n->rss_data.redirect) {
> +        return false;
> +    }
> +    filtered = vhost_net_get_features(get_vhost_net(nc->peer), features);
> +    if (filtered != features) {
> +        return false;
> +    }
> +    return true;
> +}
> +
>  static void virtio_net_set_features(VirtIODevice *vdev, uint64_t features)
>  {
>      VirtIONet *n = VIRTIO_NET(vdev);
>      Error *err = NULL;
> +    bool disable_vhost = false;
>      int i;
>  
>      if (n->mtu_bypass_backend &&
> @@ -894,13 +928,21 @@ static void virtio_net_set_features(VirtIODevice *vdev, uint64_t features)
>                                                    VIRTIO_F_VERSION_1),
>                                 virtio_has_feature(features,
>                                                    VIRTIO_NET_F_HASH_REPORT));
> -
>      n->rsc4_enabled = virtio_has_feature(features, VIRTIO_NET_F_RSC_EXT) &&
>          virtio_has_feature(features, VIRTIO_NET_F_GUEST_TSO4);
>      n->rsc6_enabled = virtio_has_feature(features, VIRTIO_NET_F_RSC_EXT) &&
>          virtio_has_feature(features, VIRTIO_NET_F_GUEST_TSO6);
>      n->rss_data.redirect = virtio_has_feature(features, VIRTIO_NET_F_RSS);
>  
> +    if (can_disable_vhost(n)) {
> +        disable_vhost = !check_vhost_features(n, features);
> +    }
> +    if (disable_vhost) {
> +        warn_report("Some of requested features aren't supported by vhost, "
> +                    "vhost is turned off till next reset");
> +        virtio_net_allow_vhost(n, false);
> +    }
> +
>      if (n->has_vnet_hdr) {
>          n->curr_guest_offloads =
>              virtio_net_guest_offloads_by_features(features);
> -- 
> 2.17.1