Add support for attaching network/bridge NICs in QEMU driver

In order to hotplug a network/bridge backed NIC, we need to first create
the tap file descriptor, add the tap interface to the bridge and then
pass the file descriptor to the qemu process using the 'getfd' monitor
command.

Once the tapfd has been accepted, we create the network backend using
host_net_add, supplying the name assigned to the tapfd. If this fails,
we need to close the tapfd in qemu using the 'closefd' monitor command.

If the version of qemu does not support the getfd/closefd monitor
commands we detect "unknown command" in the getfd reply and fail the
attach operation.

* src/qemu_driver.c: add support for tapfd based hotplug in
  qemudDomainAttachNetDevice()

src/qemu_driver.c

@@ -4929,13 +4929,15 @@ static int qemudDomainAttachUsbMassstorageDevice(virConnectPtr conn,
 }
 
 static int qemudDomainAttachNetDevice(virConnectPtr conn,
+                                      struct qemud_driver *driver,
                                       virDomainObjPtr vm,
                                       virDomainDeviceDefPtr dev,
                                       unsigned int qemuCmdFlags)
 {
     virDomainNetDefPtr net = dev->data.net;
     char *cmd = NULL, *reply = NULL, *remove_cmd = NULL;
-    int i;
+    char *tapfd_name = NULL, *tapfd_close = NULL;
+    int i, tapfd = -1;
     unsigned domain, bus, slot;
 
     if (!(qemuCmdFlags & QEMUD_CMD_FLAG_HOST_NET_ADD)) {
@@ -4946,10 +4948,16 @@ static int qemudDomainAttachNetDevice(virConnectPtr conn,
 
     if (net->type == VIR_DOMAIN_NET_TYPE_BRIDGE ||
         net->type == VIR_DOMAIN_NET_TYPE_NETWORK) {
-        qemudReportError(conn, dom, NULL, VIR_ERR_NO_SUPPORT,
-                         _("network device type '%s' cannot be attached"),
-                         virDomainNetTypeToString(net->type));
-        return -1;
+        if (vm->monitor_chr->type != VIR_DOMAIN_CHR_TYPE_UNIX) {
+            qemudReportError(conn, dom, NULL, VIR_ERR_NO_SUPPORT,
+                             _("network device type '%s' cannot be attached: "
+                               "qemu is not using a unix socket monitor"),
+                             virDomainNetTypeToString(net->type));
+            return -1;
+        }
+
+        if ((tapfd = qemudNetworkIfaceConnect(conn, driver, net, qemuCmdFlags)) < 0)
+            return -1;
     }
 
     if (VIR_REALLOC_N(vm->def->nets, vm->def->nnets+1) < 0)
@@ -4967,27 +4975,65 @@ static int qemudDomainAttachNetDevice(virConnectPtr conn,
         if (vm->def->nets[i]->vlan >= net->vlan)
             net->vlan = vm->def->nets[i]->vlan;
 
-    if (qemuBuildHostNetStr(conn, net,
-                            "host_net_add ", ' ', net->vlan, NULL, &cmd) < 0)
-        goto cleanup;
+    if (tapfd != -1) {
+        if (virAsprintf(&tapfd_name, "fd-%s", net->hostnet_name) < 0)
+            goto no_memory;
+
+        if (virAsprintf(&tapfd_close, "closefd %s", tapfd_name) < 0)
+            goto no_memory;
+
+        if (virAsprintf(&cmd, "getfd %s", tapfd_name) < 0)
+            goto no_memory;
+
+        if (qemudMonitorCommandWithFd(vm, cmd, tapfd, &reply) < 0) {
+            qemudReportError(conn, dom, NULL, VIR_ERR_OPERATION_FAILED,
+                             _("failed to pass fd to qemu with '%s'"), cmd);
+            goto cleanup;
+        }
+
+        DEBUG("%s: getfd reply: %s", vm->def->name, reply);
+
+        /* If the command isn't supported then qemu prints:
+         * unknown command: getfd" */
+        if (strstr(reply, "unknown command:")) {
+            qemudReportError(conn, dom, NULL, VIR_ERR_NO_SUPPORT,
+                             "%s",
+                             _("bridge/network interface attach not supported: "
+                               "qemu 'getfd' monitor command not available"));
+            goto cleanup;
+        }
+
+        VIR_FREE(reply);
+        VIR_FREE(cmd);
+    }
+
+    if (qemuBuildHostNetStr(conn, net, "host_net_add ", ' ',
+                            net->vlan, tapfd_name, &cmd) < 0)
+        goto try_tapfd_close;
 
     remove_cmd = NULL;
     if (net->vlan >= 0 && net->hostnet_name &&
         virAsprintf(&remove_cmd, "host_net_remove %d %s",
                     net->vlan, net->hostnet_name) < 0) {
-        goto no_memory;
+        virReportOOMError(conn);
+        goto try_tapfd_close;
     }
 
     if (qemudMonitorCommand(vm, cmd, &reply) < 0) {
         qemudReportError(conn, dom, NULL, VIR_ERR_OPERATION_FAILED,
                          _("failed to add network backend with '%s'"), cmd);
-        goto cleanup;
+        goto try_tapfd_close;
     }
 
     DEBUG("%s: host_net_add reply: %s", vm->def->name, reply);
 
     VIR_FREE(reply);
     VIR_FREE(cmd);
+    VIR_FREE(tapfd_name);
+    VIR_FREE(tapfd_close);
+    if (tapfd != -1)
+        close(tapfd);
+    tapfd = -1;
 
     if (qemuBuildNicStr(conn, net,
                         "pci_add pci_addr=auto ", ' ', net->vlan, &cmd) < 0)
@@ -5028,12 +5074,27 @@ try_remove:
         VIR_DEBUG("%s: host_net_remove reply: %s\n", vm->def->name, reply);
     goto cleanup;
 
+try_tapfd_close:
+    VIR_FREE(reply);
+
+    if (tapfd_close) {
+        if (qemudMonitorCommand(vm, tapfd_close, &reply) < 0)
+            VIR_WARN(_("Failed to close tapfd with '%s'\n"), tapfd_close);
+        else
+            VIR_DEBUG("%s: closefd: %s\n", vm->def->name, reply);
+    }
+    goto cleanup;
+
 no_memory:
     virReportOOMError(conn);
 cleanup:
     VIR_FREE(cmd);
     VIR_FREE(reply);
     VIR_FREE(remove_cmd);
+    VIR_FREE(tapfd_close);
+    VIR_FREE(tapfd_name);
+    if (tapfd != -1)
+        close(tapfd);
     return -1;
 }
 
@@ -5189,7 +5250,7 @@ static int qemudDomainAttachDevice(virDomainPtr dom,
                                     dev->data.disk->src);
         }
     } else if (dev->type == VIR_DOMAIN_DEVICE_NET) {
-        ret = qemudDomainAttachNetDevice(dom->conn, vm, dev, qemuCmdFlags);
+        ret = qemudDomainAttachNetDevice(dom->conn, driver, vm, dev, qemuCmdFlags);
     } else if (dev->type == VIR_DOMAIN_DEVICE_HOSTDEV &&
                dev->data.hostdev->mode == VIR_DOMAIN_HOSTDEV_MODE_SUBSYS &&
                dev->data.hostdev->source.subsys.type == VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_USB) {