Re: [Qemu-devel] [RFC v2 3/3] virtio-net: Add MTU feature support

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

On Thu, Nov 17, 2016 at 10:58:07PM +0100, Maxime Coquelin wrote:
> If negotiated, virtio-net gets the advised MTU from vhost-net,
> and provides it to the guest through a new virtio_net_config
> entry.
> 
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Aaron Conole <aconole@redhat.com
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>  hw/net/virtio-net.c            | 14 ++++++++++++++
>  include/hw/virtio/virtio-net.h |  1 +
>  2 files changed, 15 insertions(+)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 5009533..36843fe 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -55,6 +55,8 @@ static VirtIOFeature feature_sizes[] = {
>       .end = endof(struct virtio_net_config, status)},
>      {.flags = 1 << VIRTIO_NET_F_MQ,
>       .end = endof(struct virtio_net_config, max_virtqueue_pairs)},
> +    {.flags = 1 << VIRTIO_NET_F_MTU,
> +     .end = endof(struct virtio_net_config, mtu)},
>      {}
>  };
>  
> @@ -81,6 +83,7 @@ static void virtio_net_get_config(VirtIODevice *vdev, uint8_t *config)
>  
>      virtio_stw_p(vdev, &netcfg.status, n->status);
>      virtio_stw_p(vdev, &netcfg.max_virtqueue_pairs, n->max_queues);
> +    virtio_stw_p(vdev, &netcfg.mtu, n->mtu);
>      memcpy(netcfg.mac, n->mac, ETH_ALEN);
>      memcpy(config, &netcfg, n->config_size);
>  }
> @@ -636,6 +639,14 @@ static void virtio_net_set_features(VirtIODevice *vdev, uint64_t features)
>      } else {
>          memset(n->vlans, 0xff, MAX_VLAN >> 3);
>      }
> +
> +    if (virtio_has_feature(features, VIRTIO_NET_F_MTU)) {
> +        NetClientState *nc = qemu_get_queue(n->nic);
> +
> +        if (get_vhost_net(nc->peer)) {
> +            n->mtu = vhost_net_get_mtu(get_vhost_net(nc->peer));


This means migration breaks silently if you move from a bigger to
smaller mtu. We don't necessarily have to make it work,
but it should be detected and reported.

> +        }
> +    }
>  }
>  
>  static int virtio_net_handle_rx_mode(VirtIONet *n, uint8_t cmd,

What about non-vhost traffic? You need to ensure guest does
not get bigger packets.


> @@ -1695,6 +1706,7 @@ static void virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
>  {
>      int i, config_size = 0;
>      virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
> +    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);
>      for (i = 0; feature_sizes[i].flags != 0; i++) {
>          if (host_features & feature_sizes[i].flags) {
>              config_size = MAX(feature_sizes[i].end, config_size);
> @@ -1922,6 +1934,8 @@ static Property virtio_net_properties[] = {
>      DEFINE_PROP_STRING("tx", VirtIONet, net_conf.tx),
>      DEFINE_PROP_UINT16("rx_queue_size", VirtIONet, net_conf.rx_queue_size,
>                         VIRTIO_NET_RX_QUEUE_DEFAULT_SIZE),
> +    DEFINE_PROP_BIT("host_mtu", VirtIONet, host_features,
> +                    VIRTIO_NET_F_MTU, true),
>      DEFINE_PROP_END_OF_LIST(),
>  };
>

Cross version migration support is missing here.

  
> diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
> index 0ced975..b6394c8 100644
> --- a/include/hw/virtio/virtio-net.h
> +++ b/include/hw/virtio/virtio-net.h
> @@ -96,6 +96,7 @@ typedef struct VirtIONet {
>      QEMUTimer *announce_timer;
>      int announce_counter;
>      bool needs_vnet_hdr_swap;
> +    uint16_t mtu;
>  } VirtIONet;
>  
>  void virtio_net_set_netclient_name(VirtIONet *n, const char *name,
> -- 
> 2.7.4