Support transferring of TAP state (including open fd) through
migration stream.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 net/tap.c | 46 +++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 45 insertions(+), 1 deletion(-)

diff --git a/net/tap.c b/net/tap.c
index ad1d458521..4a752006de 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -413,6 +413,49 @@ static VHostNetState *tap_get_vhost_net(NetClientState *nc)
     return s->vhost_net;
 }
 
+static int tap_pre_load(void *opaque)
+{
+    TAPState *s = opaque;
+
+    if (s->fd != -1) {
+        error_report(
+            "TAP is already initialized and cannot receive incoming fd");
+        return -EINVAL;
+    }
+
+    return 0;
+}
+
+static int tap_post_load(void *opaque, int version_id)
+{
+    TAPState *s = opaque;
+    Error *local_err = NULL;
+
+    if (!net_tap_setup(s, -1, -1, &local_err)) {
+        error_report_err(local_err);
+        qemu_del_net_client(&s->nc);
+        return -EINVAL;
+    }
+
+    return 0;
+}
+
+static const VMStateDescription vmstate_tap = {
+    .name = "net-tap",
+    .pre_load = tap_pre_load,
+    .post_load = tap_post_load,
+    .fields = (const VMStateField[]) {
+        VMSTATE_FD(fd, TAPState),
+        VMSTATE_BOOL(using_vnet_hdr, TAPState),
+        VMSTATE_BOOL(has_ufo, TAPState),
+        VMSTATE_BOOL(has_uso, TAPState),
+        VMSTATE_BOOL(has_tunnel, TAPState),
+        VMSTATE_BOOL(enabled, TAPState),
+        VMSTATE_UINT32(host_vnet_hdr_len, TAPState),
+        VMSTATE_END_OF_LIST()
+    }
+};
+
 /* fd support */
 
 static NetClientInfo net_tap_info = {
@@ -434,6 +477,7 @@ static NetClientInfo net_tap_info = {
     .set_steering_ebpf = tap_set_steering_ebpf,
     .get_vhost_net = tap_get_vhost_net,
     .backend_connect = tap_backend_connect,
+    .backend_vmsd = &vmstate_tap,
 };
 
 static TAPState *net_tap_new(NetClientState *peer, const char *model,
@@ -800,7 +844,7 @@ static void net_init_tap_one(const NetdevTapOptions *tap, NetClientState *peer,
 
 static bool net_tap_setup(TAPState *s, int fd, int vnet_hdr, Error **errp)
 {
-    if (!net_tap_set_fd(s, fd, vnet_hdr, errp)) {
+    if (fd != -1 && !net_tap_set_fd(s, fd, vnet_hdr, errp)) {
         return false;
     }
 
-- 
2.48.1