qemu: reorganize qemuDomainChangeNet and qemuDomainChangeNetBridge

This patch resolves:

  https://bugzilla.redhat.com/show_bug.cgi?id=805071

to the extent that it can be resolved with current qemu functionality.
It attempts to detect as many situations as possible when the simple
operation of disconnecting an existing tap device from one bridge and
attaching it to another will satisfy the change requested in
virDomainUpdateDeviceFlags() for a network device. Before this patch,
that situation could only be detected if the pre-change interface
*and* the post-change interface definition were both "type='bridge'".
After this patch, it can also be detected if the before or after
interfaces are any combination of type='bridge' and type='network'
(the networks can be <forward mode='nat|route|bridge'>, as long as
they use a Linux host bridge and not macvtap connections).

This extra effort is especially useful since the recent discovery that
a netdev_del+netdev_add combo (to reconnect the network device with
completely different hostside configuration) doesn't work properly
with current qemu (1.2) unless it is accompanied by the matching
device_del+device_add - see this mailing list message for details:

  http://lists.nongnu.org/archive/html/qemu-devel/2012-10/msg02355.html

(A slight modification of the patch referenced there has been prepared
to apply on top of this patch, but won't be pushed until qemu can be
made to work with it.)

* qemuDomainChangeNet needs access to the virDomainDeviceDef that
holds the new netdef (so that it can clear out the virDomainDeviceDef
if it ends up using the NetDef to replace the original), so the
virDomainNetDefPtr arg is replaced with a virDomainDeviceDefPtr.

* qemuDomainChangeNet previously checked for *some* changes to the
interface config, but this check was by no means complete. It was also
a bit disorganized.

This refactoring of the code is (I believe) complete in its check of
all NetDef attributes that might be changed, and either returns a
failure (for changes that are simply impossible), or sets one of three
flags:

  needLinkStateChange - if the device link state needs to go up/down
  needBridgeChange    - if everything else is the same, but it needs
                        to be connected to a difference linux host
                        bridge
  needReconnect       - if the entire host side of the device needs
                        to be torn down and reconstructed (currently
                        non-working, as mentioned above)

Note that this function will refuse to make any change that requires
the *guest* side of the device to be detached (e.g. changing the PCI
address or mac address). Those would be disruptive enough to the guest
that it's reasonable to require an explicit detach/attach sequence
from the management application.

* As mentioned above, qemuDomainChangeNet also does its best to
understand when a simple change in attached bridge for the existing
tap device will work vs. the need to completely tear down/reconstruct
the host side of the device (including tap device).

This patch *does not* implement the "reconnect" code anyway - there is
a placeholder that turns that into an error. Rather, the purpose of
this patch is to replicate existing behavior with code that is ready
to have that functionality plugged in in a later patch.

* The expanded uses for qemuDomainChangeNetBridge meant that it needed
to be enhanced as well - it no longer replaces the original brname
string in olddev with the new brname; instead, it relies on the
caller to replace the *entire* olddev with newdev (since we've gone
to great lengths to assure they are functionally identical other
than the name of the bridge, this is now not only safe, but more
correct). Additionally, qemuDomainNetChangeBridge can now set the
bridge for type='network' interfaces as well as plain type='bridge'
interfaces. (Note that I had to make this change simultaneous to the
reorganization of qemuDomainChangeNet because the two are too
closely intertwined to separate).

src/qemu/qemu_driver.c

@@ -6097,7 +6097,7 @@ qemuDomainUpdateDeviceLive(virDomainObjPtr vm,
         ret = qemuDomainChangeGraphics(driver, vm, dev->data.graphics);
         break;
     case VIR_DOMAIN_DEVICE_NET:
-        ret = qemuDomainChangeNet(driver, vm, dom, dev->data.net);
+        ret = qemuDomainChangeNet(driver, vm, dom, dev);
         break;
     default:
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,

src/qemu/qemu_hotplug.c

@@ -33,6 +33,7 @@
 #include "domain_audit.h"
 #include "domain_nwfilter.h"
 #include "logging.h"
+#include "datatypes.h"
 #include "virterror_internal.h"
 #include "memory.h"
 #include "pci.h"
@@ -1209,27 +1210,88 @@ cleanup:
     return -1;
 }
 
-static virDomainNetDefPtr qemuDomainFindNet(virDomainObjPtr vm,
+static virDomainNetDefPtr *qemuDomainFindNet(virDomainObjPtr vm,
                                              virDomainNetDefPtr dev)
 {
     int i;
 
     for (i = 0; i < vm->def->nnets; i++) {
         if (virMacAddrCmp(&vm->def->nets[i]->mac, &dev->mac) == 0)
-            return vm->def->nets[i];
+            return &vm->def->nets[i];
     }
 
     return NULL;
 }
 
-static
-int qemuDomainChangeNetBridge(virDomainObjPtr vm,
+static char *
+qemuDomainNetGetBridgeName(virConnectPtr conn, virDomainNetDefPtr net)
+{
+    char *brname = NULL;
+    int actualType = virDomainNetGetActualType(net);
+
+    if (actualType == VIR_DOMAIN_NET_TYPE_BRIDGE) {
+        const char *tmpbr = virDomainNetGetActualBridgeName(net);
+        if (!tmpbr) {
+            virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                           _("interface is missing bridge name"));
+            goto cleanup;
+        }
+        /* we need a copy, not just a pointer to the original */
+        if (!(brname = strdup(tmpbr))) {
+            virReportOOMError();
+            goto cleanup;
+        }
+    } else if (actualType == VIR_DOMAIN_NET_TYPE_NETWORK) {
+        int active;
+        virErrorPtr errobj;
+        virNetworkPtr network;
+
+        if (!(network = virNetworkLookupByName(conn, net->data.network.name))) {
+            virReportError(VIR_ERR_INTERNAL_ERROR,
+                           _("Couldn't find network '%s'"),
+                           net->data.network.name);
+            goto cleanup;
+        }
+
+        active = virNetworkIsActive(network);
+        if (active == 1) {
+            brname = virNetworkGetBridgeName(network);
+        } else if (active == 0) {
+            virReportError(VIR_ERR_INTERNAL_ERROR,
+                           _("Network '%s' is not active."),
+                           net->data.network.name);
+        }
+
+        /* Make sure any above failure is preserved */
+        errobj = virSaveLastError();
+        virNetworkFree(network);
+        virSetError(errobj);
+        virFreeError(errobj);
+        goto cleanup;
+
+    }
+
+    virReportError(VIR_ERR_INTERNAL_ERROR,
+                   _("Network type %d is not supported"),
+                   virDomainNetGetActualType(net));
+cleanup:
+    return brname;
+}
+
+static int
+qemuDomainChangeNetBridge(virConnectPtr conn,
+                          virDomainObjPtr vm,
                           virDomainNetDefPtr olddev,
                           virDomainNetDefPtr newdev)
 {
     int ret = -1;
-    char *oldbridge = olddev->data.bridge.brname;
-    char *newbridge = newdev->data.bridge.brname;
+    char *oldbridge = NULL, *newbridge = NULL;
+
+    if (!(oldbridge = qemuDomainNetGetBridgeName(conn, olddev)))
+        goto cleanup;
+
+    if (!(newbridge = qemuDomainNetGetBridgeName(conn, newdev)))
+        goto cleanup;
 
     VIR_DEBUG("Change bridge for interface %s: %s -> %s",
               olddev->ifname, oldbridge, newbridge);
@@ -1237,23 +1299,19 @@ int qemuDomainChangeNetBridge(virDomainObjPtr vm,
     if (virNetDevExists(newbridge) != 1) {
         virReportError(VIR_ERR_OPERATION_FAILED,
                        _("bridge %s doesn't exist"), newbridge);
-        return -1;
+        goto cleanup;
     }
 
     if (oldbridge) {
         ret = virNetDevBridgeRemovePort(oldbridge, olddev->ifname);
         virDomainAuditNet(vm, olddev, NULL, "detach", ret == 0);
         if (ret < 0)
-            return -1;
+            goto cleanup;
     }
 
-    /* move newbridge into olddev now so Audit log is correct */
-    olddev->data.bridge.brname = newbridge;
     ret = virNetDevBridgeAddPort(newbridge, olddev->ifname);
-    virDomainAuditNet(vm, NULL, olddev, "attach", ret == 0);
+    virDomainAuditNet(vm, NULL, newdev, "attach", ret == 0);
     if (ret < 0) {
-        /* restore oldbridge to olddev */
-        olddev->data.bridge.brname = oldbridge;
         ret = virNetDevBridgeAddPort(oldbridge, olddev->ifname);
         virDomainAuditNet(vm, NULL, olddev, "attach", ret == 0);
         if (ret < 0) {
@@ -1261,12 +1319,14 @@ int qemuDomainChangeNetBridge(virDomainObjPtr vm,
                            _("unable to recover former state by adding port "
                              "to bridge %s"), oldbridge);
         }
-        return -1;
+        goto cleanup;
     }
-    /* oldbridge no longer needed, and newbridge moved to olddev */
+    /* caller will replace entire olddev with newdev in domain nets list */
+    ret = 0;
+cleanup:
     VIR_FREE(oldbridge);
-    newdev->data.bridge.brname = NULL;
-    return 0;
+    VIR_FREE(newbridge);
+    return ret;
 }
 
 int qemuDomainChangeNetLinkState(struct qemud_driver *driver,
@@ -1300,124 +1360,357 @@ cleanup:
     return ret;
 }
 
-int qemuDomainChangeNet(struct qemud_driver *driver,
+int
+qemuDomainChangeNet(struct qemud_driver *driver,
                     virDomainObjPtr vm,
-                        virDomainPtr dom ATTRIBUTE_UNUSED,
-                        virDomainNetDefPtr dev)
-
+                    virDomainPtr dom,
+                    virDomainDeviceDefPtr dev)
 {
-    virDomainNetDefPtr olddev = qemuDomainFindNet(vm, dev);
-    int ret = 0;
+    virDomainNetDefPtr newdev = dev->data.net;
+    virDomainNetDefPtr *devslot = qemuDomainFindNet(vm, newdev);
+    virDomainNetDefPtr olddev;
+    int oldType, newType;
+    bool needReconnect = false;
+    bool needBridgeChange = false;
+    bool needLinkStateChange = false;
+    bool needReplaceDevDef = false;
+    int ret = -1;
 
-    if (!olddev) {
+    if (!devslot || !(olddev = *devslot)) {
         virReportError(VIR_ERR_NO_SUPPORT, "%s",
                        _("cannot find existing network device to modify"));
-        return -1;
+        goto cleanup;
     }
 
-    if (olddev->type != dev->type) {
+    oldType = virDomainNetGetActualType(olddev);
+    if (oldType == VIR_DOMAIN_NET_TYPE_HOSTDEV) {
+        /* no changes are possible to a type='hostdev' interface */
+        virReportError(VIR_ERR_NO_SUPPORT,
+                       _("cannot change config of '%s' network type"),
+                       virDomainNetTypeToString(oldType));
+        goto cleanup;
+    }
+
+    /* Check individual attributes for changes that can't be done to a
+     * live netdev. These checks *mostly* go in order of the
+     * declarations in virDomainNetDef in order to assure nothing is
+     * omitted. (exceptiong where noted in comments - in particular,
+     * some things require that a new "actual device" be allocated
+     * from the network driver first, but we delay doing that until
+     * after we've made as many other checks as possible)
+     */
+
+    /* type: this can change (with some restrictions), but the actual
+     * type of the new device connection isn't known until after we
+     * allocate the "actual" device.
+     */
+
+    if (virMacAddrCmp(&olddev->mac, &newdev->mac)) {
+        char oldmac[VIR_MAC_STRING_BUFLEN], newmac[VIR_MAC_STRING_BUFLEN];
+
+        virReportError(VIR_ERR_NO_SUPPORT,
+                       _("cannot change network interface mac address "
+                         "from %s to %s"),
+                       virMacAddrFormat(&olddev->mac, oldmac),
+                       virMacAddrFormat(&newdev->mac, newmac));
+        goto cleanup;
+    }
+
+    if (STRNEQ_NULLABLE(olddev->model, newdev->model)) {
+        virReportError(VIR_ERR_NO_SUPPORT,
+                       _("cannot modify network device model from %s to %s"),
+                       olddev->model ? olddev->model : "(default)",
+                       newdev->model ? newdev->model : "(default)");
+        goto cleanup;
+    }
+
+    if (olddev->model && STREQ(olddev->model, "virtio") &&
+        (olddev->driver.virtio.name != newdev->driver.virtio.name ||
+         olddev->driver.virtio.txmode != newdev->driver.virtio.txmode ||
+         olddev->driver.virtio.ioeventfd != newdev->driver.virtio.ioeventfd ||
+         olddev->driver.virtio.event_idx != newdev->driver.virtio.event_idx)) {
         virReportError(VIR_ERR_NO_SUPPORT, "%s",
-                       _("cannot change network interface type"));
-        return -1;
+                       _("cannot modify virtio network device driver attributes"));
+        goto cleanup;
+    }
+
+    /* data: this union will be examined later, after allocating new actualdev */
+    /* virtPortProfile: will be examined later, after allocating new actualdev */
+
+    if (olddev->tune.sndbuf_specified != newdev->tune.sndbuf_specified ||
+        olddev->tune.sndbuf != newdev->tune.sndbuf) {
+        needReconnect = true;
+    }
+
+    if (STRNEQ_NULLABLE(olddev->script, newdev->script)) {
+        virReportError(VIR_ERR_NO_SUPPORT, "%s",
+                       _("cannot modify network device script attribute"));
+        goto cleanup;
     }
 
-    if (!virNetDevVPortProfileEqual(olddev->virtPortProfile, dev->virtPortProfile)) {
+    /* ifname: check if it's set in newdev. If not, retain the autogenerated one */
+    if (!(newdev->ifname ||
+          (newdev->ifname = strdup(olddev->ifname)))) {
+        virReportOOMError();
+        goto cleanup;
+    }
+    if (STRNEQ_NULLABLE(olddev->ifname, newdev->ifname)) {
         virReportError(VIR_ERR_NO_SUPPORT, "%s",
-                       _("cannot change <virtualport> settings"));
+                       _("cannot modify network device tap name"));
+        goto cleanup;
     }
 
-    switch (olddev->type) {
+    /* info: if newdev->info is empty, fill it in from olddev,
+     * otherwise verify that it matches - nothing is allowed to
+     * change. (There is no helper function to do this, so
+     * individually check the few feidls of virDomainDeviceInfo that
+     * are relevant in this case).
+     */
+    if (!virDomainDeviceAddressIsValid(&newdev->info,
+                                       VIR_DOMAIN_DEVICE_ADDRESS_TYPE_PCI) &&
+        virDomainDeviceInfoCopy(&newdev->info, &olddev->info) < 0) {
+        goto cleanup;
+    }
+    if (!virDevicePCIAddressEqual(&olddev->info.addr.pci,
+                                  &newdev->info.addr.pci)) {
+        virReportError(VIR_ERR_NO_SUPPORT, "%s",
+                       _("cannot modify network device guest PCI address"));
+        goto cleanup;
+    }
+    /* grab alias from olddev if not set in newdev */
+    if (!(newdev->info.alias ||
+          (newdev->info.alias = strdup(olddev->info.alias)))) {
+        virReportOOMError();
+        goto cleanup;
+    }
+    if (STRNEQ_NULLABLE(olddev->info.alias, newdev->info.alias)) {
+        virReportError(VIR_ERR_NO_SUPPORT, "%s",
+                       _("cannot modify network device alias"));
+        goto cleanup;
+    }
+    if (olddev->info.rombar != newdev->info.rombar) {
+        virReportError(VIR_ERR_NO_SUPPORT, "%s",
+                       _("cannot modify network device rom bar setting"));
+        goto cleanup;
+    }
+    if (STRNEQ_NULLABLE(olddev->info.romfile, newdev->info.romfile)) {
+        virReportError(VIR_ERR_NO_SUPPORT, "%s",
+                       _("cannot modify network rom file"));
+        goto cleanup;
+    }
+    if (olddev->info.bootIndex != newdev->info.bootIndex) {
+        virReportError(VIR_ERR_NO_SUPPORT, "%s",
+                       _("cannot modify network device boot index setting"));
+        goto cleanup;
+    }
+    /* (end of device info checks) */
+
+    if (STRNEQ_NULLABLE(olddev->filter, newdev->filter))
+        needReconnect = true;
+
+    /* bandwidth can be modified, and will be checked later */
+    /* vlan can be modified, and will be checked later */
+    /* linkstate can be modified */
+
+    /* allocate new actual device to compare to old - we will need to
+     * free it if we fail for any reason
+     */
+    if (newdev->type == VIR_DOMAIN_NET_TYPE_NETWORK &&
+        networkAllocateActualDevice(newdev) < 0) {
+        goto cleanup;
+    }
+
+    newType = virDomainNetGetActualType(newdev);
+
+    if (newType == VIR_DOMAIN_NET_TYPE_HOSTDEV) {
+        /* can't turn it into a type='hostdev' interface */
+        virReportError(VIR_ERR_NO_SUPPORT,
+                       _("cannot change network interface type to '%s'"),
+                       virDomainNetTypeToString(newType));
+        goto cleanup;
+    }
+
+    if (olddev->type == newdev->type && oldType == newType) {
+
+        /* if type hasn't changed, check the relevant fields for the type */
+        switch (newdev->type) {
         case VIR_DOMAIN_NET_TYPE_USER:
             break;
 
         case VIR_DOMAIN_NET_TYPE_ETHERNET:
-        if (STRNEQ_NULLABLE(olddev->data.ethernet.dev, dev->data.ethernet.dev) ||
-            STRNEQ_NULLABLE(olddev->script, dev->script) ||
-            STRNEQ_NULLABLE(olddev->data.ethernet.ipaddr, dev->data.ethernet.ipaddr)) {
-            virReportError(VIR_ERR_NO_SUPPORT, "%s",
-                           _("cannot modify ethernet network device configuration"));
-            return -1;
+            if (STRNEQ_NULLABLE(olddev->data.ethernet.dev,
+                                newdev->data.ethernet.dev) ||
+                STRNEQ_NULLABLE(olddev->data.ethernet.ipaddr,
+                                newdev->data.ethernet.ipaddr)) {
+                needReconnect = true;
             }
         break;
 
         case VIR_DOMAIN_NET_TYPE_SERVER:
         case VIR_DOMAIN_NET_TYPE_CLIENT:
         case VIR_DOMAIN_NET_TYPE_MCAST:
-        if (STRNEQ_NULLABLE(olddev->data.socket.address, dev->data.socket.address) ||
-            olddev->data.socket.port != dev->data.socket.port) {
-            virReportError(VIR_ERR_NO_SUPPORT, "%s",
-                           _("cannot modify network socket device configuration"));
-            return -1;
+            if (STRNEQ_NULLABLE(olddev->data.socket.address,
+                                newdev->data.socket.address) ||
+                olddev->data.socket.port != newdev->data.socket.port) {
+                needReconnect = true;
             }
             break;
 
         case VIR_DOMAIN_NET_TYPE_NETWORK:
-        if (STRNEQ_NULLABLE(olddev->data.network.name, dev->data.network.name) ||
-            STRNEQ_NULLABLE(olddev->data.network.portgroup, dev->data.network.portgroup)) {
-            virReportError(VIR_ERR_NO_SUPPORT, "%s",
-                           _("cannot modify network device configuration"));
-            return -1;
+            if (STRNEQ(olddev->data.network.name, newdev->data.network.name)) {
+                if (virDomainNetGetActualVirtPortProfile(newdev))
+                    needReconnect = true;
+                else
+                    needBridgeChange = true;
             }
-
+            /* other things handled in common code directly below this switch */
             break;
 
         case VIR_DOMAIN_NET_TYPE_BRIDGE:
-       /* allow changing brname */
+            /* all handled in bridge name checked in common code below */
             break;
 
         case VIR_DOMAIN_NET_TYPE_INTERNAL:
-        if (STRNEQ_NULLABLE(olddev->data.internal.name, dev->data.internal.name)) {
-            virReportError(VIR_ERR_NO_SUPPORT, "%s",
-                           _("cannot modify internal network device configuration"));
-            return -1;
+            if (STRNEQ_NULLABLE(olddev->data.internal.name,
+                                newdev->data.internal.name)) {
+                needReconnect = true;
             }
             break;
 
         case VIR_DOMAIN_NET_TYPE_DIRECT:
-        if (STRNEQ_NULLABLE(olddev->data.direct.linkdev, dev->data.direct.linkdev) ||
-            olddev->data.direct.mode != dev->data.direct.mode) {
-            virReportError(VIR_ERR_NO_SUPPORT, "%s",
-                           _("cannot modify direct network device configuration"));
-            return -1;
-        }
+            /* all handled in common code directly below this switch */
             break;
 
         default:
-        virReportError(VIR_ERR_INTERNAL_ERROR,
+            virReportError(VIR_ERR_NO_SUPPORT,
                            _("unable to change config on '%s' network type"),
-                       virDomainNetTypeToString(dev->type));
+                           virDomainNetTypeToString(newdev->type));
             break;
 
         }
+    } else {
+        /* interface type has changed. There are a few special cases
+         * where this can only require a minor (or even no) change,
+         * but in most cases we need to do a full reconnection.
+         *
+         * If we switch (in either direction) between type='bridge'
+         * and type='network' (for a traditional managed virtual
+         * network that uses a host bridge, i.e. forward
+         * mode='route|nat'), we just need to change the bridge.
+         */
+        if ((oldType == VIR_DOMAIN_NET_TYPE_NETWORK &&
+             newType == VIR_DOMAIN_NET_TYPE_BRIDGE) ||
+            (oldType == VIR_DOMAIN_NET_TYPE_BRIDGE &&
+             newType == VIR_DOMAIN_NET_TYPE_NETWORK)) {
 
-    /* all other unmodifiable parameters */
-    if (STRNEQ_NULLABLE(olddev->model, dev->model) ||
-        STRNEQ_NULLABLE(olddev->filter, dev->filter)) {
-        virReportError(VIR_ERR_NO_SUPPORT, "%s",
-                       _("cannot modify network device configuration"));
-        return -1;
+            needBridgeChange = true;
+
+        } else if (oldType == VIR_DOMAIN_NET_TYPE_DIRECT &&
+                   newType == VIR_DOMAIN_NET_TYPE_DIRECT) {
+
+            /* this is the case of switching from type='direct' to
+             * type='network' for a network that itself uses direct
+             * (macvtap) devices. If the physical device and mode are
+             * the same, this doesn't require any actual setup
+             * change. If the physical device or mode *does* change,
+             * that will be caught in the common section below */
+
+        } else {
+
+            /* for all other combinations, we'll need a full reconnect */
+            needReconnect = true;
+
+        }
     }
 
-    /* check if device name has been set, if no, retain the autogenerated one */
-    if (dev->ifname &&
-        STRNEQ_NULLABLE(olddev->ifname, dev->ifname)) {
-        virReportError(VIR_ERR_NO_SUPPORT, "%s",
-                       _("cannot modify network device configuration"));
-        return -1;
+    /* now several things that are in multiple (but not all)
+     * different types, and can be safely compared even for those
+     * cases where they don't apply to a particular type.
+     */
+    if (STRNEQ_NULLABLE(virDomainNetGetActualBridgeName(olddev),
+                        virDomainNetGetActualBridgeName(newdev))) {
+        if (virDomainNetGetActualVirtPortProfile(newdev))
+            needReconnect = true;
+        else
+            needBridgeChange = true;
     }
 
-    if (olddev->type == VIR_DOMAIN_NET_TYPE_BRIDGE
-        && STRNEQ_NULLABLE(olddev->data.bridge.brname,
-                           dev->data.bridge.brname)) {
-        if ((ret = qemuDomainChangeNetBridge(vm, olddev, dev)) < 0)
-            return ret;
+    if (STRNEQ_NULLABLE(virDomainNetGetActualDirectDev(olddev),
+                        virDomainNetGetActualDirectDev(newdev)) ||
+        virDomainNetGetActualDirectMode(olddev) != virDomainNetGetActualDirectMode(olddev) ||
+        !virNetDevVPortProfileEqual(virDomainNetGetActualVirtPortProfile(olddev),
+                                    virDomainNetGetActualVirtPortProfile(newdev)) ||
+        !virNetDevBandwidthEqual(virDomainNetGetActualBandwidth(olddev),
+                                 virDomainNetGetActualBandwidth(newdev)) ||
+        !virNetDevVlanEqual(virDomainNetGetActualVlan(olddev),
+                            virDomainNetGetActualVlan(newdev))) {
+        needReconnect = true;
     }
 
-    if (olddev->linkstate != dev->linkstate) {
-        if ((ret = qemuDomainChangeNetLinkState(driver, vm, olddev, dev->linkstate)) < 0)
-            return ret;
+    if (olddev->linkstate != newdev->linkstate)
+        needLinkStateChange = true;
+
+    /* FINALLY - actually perform the required actions */
+
+    if (needReconnect) {
+        virReportError(VIR_ERR_NO_SUPPORT,
+                       _("unable to change config on '%s' network type"),
+                       virDomainNetTypeToString(newdev->type));
+        goto cleanup;
+    }
+
+    if (needBridgeChange) {
+        if (qemuDomainChangeNetBridge(dom->conn, vm, olddev, newdev) < 0)
+            goto cleanup;
+        /* we successfully switched to the new bridge, and we've
+         * determined that the rest of newdev is equivalent to olddev,
+         * so move newdev into place, so that the  */
+        needReplaceDevDef = true;
+    }
+
+    if (needLinkStateChange &&
+        qemuDomainChangeNetLinkState(driver, vm, olddev, newdev->linkstate) < 0) {
+        goto cleanup;
+    }
+
+    if (needReplaceDevDef) {
+        /* the changes above warrant replacing olddev with newdev in
+         * the domain's nets list.
+         */
+        networkReleaseActualDevice(olddev);
+        virDomainNetDefFree(olddev);
+        /* move newdev into the nets list, and NULL it out from the
+         * virDomainDeviceDef that we were given so that the caller
+         * won't delete it on return.
+         */
+        *devslot = newdev;
+        newdev = dev->data.net = NULL;
+        dev->type = VIR_DOMAIN_DEVICE_NONE;
     }
 
+    ret = 0;
+cleanup:
+    /* When we get here, we will be in one of these two states:
+     *
+     * 1) newdev has been moved into the domain's list of nets and
+     *    newdev set to NULL, and dev->data.net will be NULL (and
+     *    dev->type is NONE). olddev will have been completely
+     *    released and freed. (aka success) In this case no extra
+     *    cleanup is needed.
+     *
+     * 2) newdev has *not* been moved into the domain's list of nets,
+     *    and dev->data.net == newdev (and dev->type == NET). In this *
+     *    case, we need to at least release the "actual device" from *
+     *    newdev (the caller will free dev->data.net a.k.a. newdev, and
+     *    the original olddev is still in used)
+     *
+     * Note that case (2) isn't necessarily a failure. It may just be
+     * that the changes were minor enough that we didn't need to
+     * replace the entire device object.
+     */
+    if (newdev)
+        networkReleaseActualDevice(newdev);
+
     return ret;
 }
 

src/qemu/qemu_hotplug.h

 /*
  * qemu_hotplug.h: QEMU device hotplug management
  *
- * Copyright (C) 2006-2007, 2009-2011 Red Hat, Inc.
+ * Copyright (C) 2006-2007, 2009-2012 Red Hat, Inc.
  * Copyright (C) 2006 Daniel P. Berrange
  *
  * This library is free software; you can redistribute it and/or
@@ -77,7 +77,7 @@ int qemuDomainChangeGraphicsPasswords(struct qemud_driver *driver,
 int qemuDomainChangeNet(struct qemud_driver *driver,
                         virDomainObjPtr vm,
                         virDomainPtr dom,
-                        virDomainNetDefPtr dev);
+                        virDomainDeviceDefPtr dev);
 int qemuDomainChangeNetLinkState(struct qemud_driver *driver,
                                  virDomainObjPtr vm,
                                  virDomainNetDefPtr dev,