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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 11/17/2016 11:38 PM, Michael S. Tsirkin wrote:
> 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.

Good point.
I planned to investigate migration case while looking deeper at
vhost-user versionning topic.

Moving from bigger to smaller MTU is indeed a problem, because the
backend may not support received buffer size.

Isn't also a problem when moving from smaller to bigger MTU, as it no
more respect the contract old host negotiated with the guest?

>
>> +        }
>> +    }
>>  }
>>
>>  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.
Ok, do you confirm checking in virtio_net_receive() that buffer size is
smaller or equal than MTU is enough?

>
>
>> @@ -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.
Sorry, I'm not sure to understand what you expect here.
Could you please provide more details?

>
>> 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

Thanks,
Maxime