To implement backend-transfer migration in virtio-net in the next
commit, we need a generic API to migrate net backend. Here is it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 include/net/net.h |  4 ++++
 net/net.c         | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/include/net/net.h b/include/net/net.h
index 45bc86fc86..aa34043b1a 100644
--- a/include/net/net.h
+++ b/include/net/net.h
@@ -5,6 +5,7 @@
 #include "qapi/qapi-types-net.h"
 #include "net/queue.h"
 #include "hw/core/qdev-properties-system.h"
+#include "migration/vmstate.h"
 
 #define MAC_FMT "%02X:%02X:%02X:%02X:%02X:%02X"
 #define MAC_ARG(x) ((uint8_t *)(x))[0], ((uint8_t *)(x))[1], \
@@ -110,6 +111,7 @@ typedef struct NetClientInfo {
     SetSteeringEBPF *set_steering_ebpf;
     NetCheckPeerType *check_peer_type;
     GetVHostNet *get_vhost_net;
+    const VMStateDescription *backend_vmsd;
 } NetClientInfo;
 
 struct NetClientState {
@@ -354,4 +356,6 @@ static inline bool net_peer_needs_padding(NetClientState *nc)
   return nc->peer && !nc->peer->do_not_pad;
 }
 
+extern const VMStateInfo vmstate_net_peer_backend;
+
 #endif
diff --git a/net/net.c b/net/net.c
index a176936f9b..58e11b9597 100644
--- a/net/net.c
+++ b/net/net.c
@@ -58,6 +58,7 @@
 #include "qapi/string-output-visitor.h"
 #include "qapi/qobject-input-visitor.h"
 #include "standard-headers/linux/virtio_net.h"
+#include "migration/vmstate.h"
 
 /* Net bridge is currently not supported for W32. */
 #if !defined(_WIN32)
@@ -2173,3 +2174,49 @@ int net_fill_rstate(SocketReadState *rs, const uint8_t *buf, int size)
     assert(size == 0);
     return 0;
 }
+
+static int get_peer_backend(QEMUFile *f, void *pv, size_t size,
+                       const VMStateField *field)
+{
+    NetClientState *nc = pv;
+    Error *local_err = NULL;
+    int ret;
+
+    if (!nc->peer) {
+        return -EINVAL;
+    }
+    nc = nc->peer;
+
+    ret = vmstate_load_state(f, nc->info->backend_vmsd, nc, 0, &local_err);
+    if (ret < 0) {
+        error_report_err(local_err);
+    }
+
+    return ret;
+}
+
+static int put_peer_backend(QEMUFile *f, void *pv, size_t size,
+                       const VMStateField *field, JSONWriter *vmdesc)
+{
+    NetClientState *nc = pv;
+    Error *local_err = NULL;
+    int ret;
+
+    if (!nc->peer) {
+        return -EINVAL;
+    }
+    nc = nc->peer;
+
+    ret = vmstate_save_state(f, nc->info->backend_vmsd, nc, 0, &local_err);
+    if (ret < 0) {
+        error_report_err(local_err);
+    }
+
+    return ret;
+}
+
+const VMStateInfo vmstate_net_peer_backend = {
+    .name = "virtio-net-nic-nc-backend",
+    .get = get_peer_backend,
+    .put = put_peer_backend,
+};
-- 
2.52.0