net: add qemu_{configure,create}_nic_device(), qemu_find_nic_info()

Most code which directly accesses nd_table[] and nb_nics uses them for
one of two things. Either "I have created a NIC device and I'd like a
configuration for it", or "I will create a NIC device *if* there is a
configuration for it".  With some variants on the theme around whether
they actually *check* if the model specified in the configuration is
the right one.

Provide functions which perform both of those, allowing platforms to
be a little more consistent and as a step towards making nd_table[]
and nb_nics private to the net code.

One might argue that platforms ought to be consistent about whether
they create the unconfigured devices or not, but making significant
user-visible changes is explicitly *not* the intent right now.

The new functions leave the 'model' field of the NICInfo as NULL after
using it for the default NIC model, unlike the qemu_check_nic_model()
function which does set nd->model to match default_model explicitly.
This is acceptable because there is no code which consumes nd->model
except this NIC-matching code in net/net.c, and no reasonable excuse
for any code wanting to use nd->model in future.

Also export the qemu_find_nic_info() helper, as some platforms have
special cases they need to handle.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>

include/net/net.h

@@ -207,7 +207,47 @@ int qemu_show_nic_models(const char *arg, const char *const *models);
 void qemu_check_nic_model(NICInfo *nd, const char *model);
 int qemu_find_nic_model(NICInfo *nd, const char * const *models,
                         const char *default_model);
+/**
+ * qemu_find_nic_info: Obtain NIC configuration information
+ * @typename: Name of device object type
+ * @match_default: Match NIC configurations with no model specified
+ * @alias: Additional model string to match (for user convenience and
+ *         backward compatibility).
+ *
+ * Search for a NIC configuration matching the NIC model constraints.
+ */
+NICInfo *qemu_find_nic_info(const char *typename, bool match_default,
+                            const char *alias);
+/**
+ * qemu_configure_nic_device: Apply NIC configuration to a given device
+ * @dev: Network device to be configured
+ * @match_default: Match NIC configurations with no model specified
+ * @alias: Additional model string to match
+ *
+ * Search for a NIC configuration for the provided device, using the
+ * additionally specified matching constraints. If found, apply the
+ * configuration using qdev_set_nic_properties() and return %true.
+ *
+ * This is used by platform code which creates the device anyway,
+ * regardless of whether there is a configuration for it. This tends
+ * to be platforms which ignore `--nodefaults` and create net devices
+ * anyway, for example because the Ethernet device on that board is
+ * always physically present.
+ */
+bool qemu_configure_nic_device(DeviceState *dev, bool match_default,
+                               const char *alias);
 
+/**
+ * qemu_create_nic_device: Create a NIC device if a configuration exists for it
+ * @typename: Object typename of network device
+ * @match_default: Match NIC configurations with no model specified
+ * @alias: Additional model string to match
+ *
+ * Search for a NIC configuration for the provided device type. If found,
+ * create an object of the corresponding type and return it.
+ */
+DeviceState *qemu_create_nic_device(const char *typename, bool match_default,
+                                    const char *alias);
 void print_net_client(Monitor *mon, NetClientState *nc);
 void net_socket_rs_init(SocketReadState *rs,
                         SocketReadStateFinalize *finalize,

net/net.c

@@ -1087,6 +1087,57 @@ static int net_init_nic(const Netdev *netdev, const char *name,
     return idx;
 }
 
+NICInfo *qemu_find_nic_info(const char *typename, bool match_default,
+                            const char *alias)
+{
+    NICInfo *nd;
+    int i;
+
+    for (i = 0; i < nb_nics; i++) {
+        nd = &nd_table[i];
+
+        if (!nd->used || nd->instantiated) {
+            continue;
+        }
+
+        if ((match_default && !nd->model) || !g_strcmp0(nd->model, typename)
+            || (alias && !g_strcmp0(nd->model, alias))) {
+            return nd;
+        }
+    }
+    return NULL;
+}
+
+
+/* "I have created a device. Please configure it if you can" */
+bool qemu_configure_nic_device(DeviceState *dev, bool match_default,
+                               const char *alias)
+{
+    NICInfo *nd = qemu_find_nic_info(object_get_typename(OBJECT(dev)),
+                                     match_default, alias);
+
+    if (nd) {
+        qdev_set_nic_properties(dev, nd);
+        return true;
+    }
+    return false;
+}
+
+/* "Please create a device, if you have a configuration for it" */
+DeviceState *qemu_create_nic_device(const char *typename, bool match_default,
+                                    const char *alias)
+{
+    NICInfo *nd = qemu_find_nic_info(typename, match_default, alias);
+    DeviceState *dev;
+
+    if (!nd) {
+        return NULL;
+    }
+
+    dev = qdev_new(typename);
+    qdev_set_nic_properties(dev, nd);
+    return dev;
+}
 
 static int (* const net_client_init_fun[NET_CLIENT_DRIVER__MAX])(
     const Netdev *netdev,