net: implement UDP tunnel features offloading

When any host or guest GSO over UDP tunnel offload is enabled the
virtio net header includes the additional tunnel-related fields,
update the size accordingly.

Push the GSO over UDP tunnel offloads all the way down to the tap
device extending the newly introduced NetFeatures struct, and
eventually enable the associated features.

As per virtio specification, to convert features bit to offload bit,
map the extended features into the reserved range.

Finally, make the vhost backend aware of the exact header layout, to
copy it correctly. The tunnel-related field are present if either
the guest or the host negotiated any UDP tunnel related feature:
add them to the kernel supported features list, to allow qemu
transfer to the backend the needed information.

Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <093b4bc68368046bffbcab2202227632d6e4e83b.1758549625.git.pabeni@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

net/net.c

@@ -575,7 +575,8 @@ void qemu_set_vnet_hdr_len(NetClientState *nc, int len)
 
     assert(len == sizeof(struct virtio_net_hdr_mrg_rxbuf) ||
            len == sizeof(struct virtio_net_hdr) ||
-           len == sizeof(struct virtio_net_hdr_v1_hash));
+           len == sizeof(struct virtio_net_hdr_v1_hash) ||
+           len == sizeof(struct virtio_net_hdr_v1_hash_tunnel));
 
     nc->vnet_hdr_len = len;
     nc->info->set_vnet_hdr_len(nc, len);

net/tap-linux.c

@@ -284,6 +284,12 @@ void tap_fd_set_offload(int fd, const NetOffloads *ol)
         if (ol->uso6) {
             offload |= TUN_F_USO6;
         }
+        if (ol->tnl) {
+            offload |= TUN_F_UDP_TUNNEL_GSO;
+        }
+        if (ol->tnl_csum) {
+            offload |= TUN_F_UDP_TUNNEL_GSO_CSUM;
+        }
     }
 
     if (ioctl(fd, TUNSETOFFLOAD, offload) != 0) {

net/tap.c

@@ -62,6 +62,8 @@ static const int kernel_feature_bits[] = {
     VIRTIO_F_NOTIFICATION_DATA,
     VIRTIO_NET_F_RSC_EXT,
     VIRTIO_NET_F_HASH_REPORT,
+    VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO,
+    VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO,
     VHOST_INVALID_FEATURE_BIT
 };