[Qemu-devel] [RFC 4/4] virtio-net: Add MTU feature support

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

If negociated, 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            | 16 ++++++++++++++++
 include/hw/virtio/virtio-net.h |  1 +
 2 files changed, 17 insertions(+)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 01f1351..0974d87 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -50,6 +50,8 @@ 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)},
     {}
 };
 
@@ -74,6 +76,10 @@ static void virtio_net_get_config(VirtIODevice *vdev, uint8_t *config)
     VirtIONet *n = VIRTIO_NET(vdev);
     struct virtio_net_config netcfg;
 
+    if (virtio_vdev_has_feature(vdev, VIRTIO_NET_F_MTU)) {
+        virtio_stw_p(vdev, &netcfg.mtu, n->mtu);
+    }
+
     virtio_stw_p(vdev, &netcfg.status, n->status);
     virtio_stw_p(vdev, &netcfg.max_virtqueue_pairs, n->max_queues);
     memcpy(netcfg.mac, n->mac, ETH_ALEN);
@@ -526,6 +532,7 @@ static uint64_t virtio_net_get_features(VirtIODevice *vdev, uint64_t features,
     features |= n->host_features;
 
     virtio_add_feature(&features, VIRTIO_NET_F_MAC);
+    virtio_add_feature(&features, VIRTIO_NET_F_MTU);
 
     if (!peer_has_vnet_hdr(n)) {
         virtio_clear_feature(&features, VIRTIO_NET_F_CSUM);
@@ -627,6 +634,14 @@ 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));
+        }
+    }
 }
 
 static int virtio_net_handle_rx_mode(VirtIONet *n, uint8_t cmd,
@@ -1688,6 +1703,7 @@ 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);
diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
index 91ed97c..13b2a8d 100644
--- a/include/hw/virtio/virtio-net.h
+++ b/include/hw/virtio/virtio-net.h
@@ -95,6 +95,7 @@ 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