net: Introduce NetClientInfo.check_peer_type()

Some network backends (vhost-user and vhost-vdpa) work only with
specific devices. At startup, they second guess what the command line
option handling will do and error out if they think a non-virtio device
will attach to them.

This second guessing is not only ugly, it can lead to wrong error
messages ('-device floppy,netdev=foo' should complain about an unknown
property, not about the wrong kind of network device being attached) and
completely ignores hotplugging.

Add a callback where backends can check compatibility with a device when
it actually tries to attach, even on hotplug.

Signed-off-by: Kevin Wolf <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Damien Hedde <[email protected]>
Acked-by: Jason Wang <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Tested-by: Peter Krempa <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

Author: kevmw

hw/core/qdev-properties-system.c

@@ -431,6 +431,12 @@ static void set_netdev(Object *obj, Visitor *v, const char *name,
             goto out;
         }
 
+        if (peers[i]->info->check_peer_type) {
+            if (!peers[i]->info->check_peer_type(peers[i], obj->class, errp)) {
+                goto out;
+            }
+        }
+
         ncs[i] = peers[i];
         ncs[i]->queue_index = i;
     }

include/net/net.h

@@ -62,6 +62,7 @@ typedef struct SocketReadState SocketReadState;
 typedef void (SocketReadStateFinalize)(SocketReadState *rs);
 typedef void (NetAnnounce)(NetClientState *);
 typedef bool (SetSteeringEBPF)(NetClientState *, int);
+typedef bool (NetCheckPeerType)(NetClientState *, ObjectClass *, Error **);
 
 typedef struct NetClientInfo {
     NetClientDriver type;
@@ -84,6 +85,7 @@ typedef struct NetClientInfo {
     SetVnetBE *set_vnet_be;
     NetAnnounce *announce;
     SetSteeringEBPF *set_steering_ebpf;
+    NetCheckPeerType *check_peer_type;
 } NetClientInfo;
 
 struct NetClientState {