Revert "qemu: propagate bridge MTU into qemu "host_mtu" option"

This reverts commit 2841e675.

It turns out that adding the host_mtu field to the PCI capabilities in
the guest bumps the length of PCI capabilities beyond the 32 byte
boundary, so the virtio-net device gets 64 bytes of ioport space
instead of 32, which offsets the address of all the other following
devices. Migration doesn't work very well when the location and length
of PCI capabilities of devices is changed between source and
destination.

This means that we need to make sure that the absence/presence of
host_mtu on the qemu commandline always matches between source and
destination, which means that we need to make setting of host_mtu an
opt-in thing (it can't happen automatically when the bridge being used
has a non-default MTU, which is what commit 2841e675 implemented).

I do want to re-implement this feature with an <mtu auto='on'/>
setting, but probably won't backport that to any stable branches, so
I'm first reverting the original commit, and that revert can be pushed
to the few releases that have been made since the original (3.1.0 -
3.3.0)

Resolves: https://bugzilla.redhat.com/1449346

src/qemu/qemu_command.c

@@ -3633,8 +3633,7 @@ qemuBuildNicDevStr(virDomainDefPtr def,
                    int vlan,
                    unsigned int bootindex,
                    size_t vhostfdSize,
-                   virQEMUCapsPtr qemuCaps,
-                   unsigned int mtu)
+                   virQEMUCapsPtr qemuCaps)
 {
     virBuffer buf = VIR_BUFFER_INITIALIZER;
     const char *nic = net->model;
@@ -3758,23 +3757,13 @@ qemuBuildNicDevStr(virDomainDefPtr def,
         virBufferAsprintf(&buf, ",rx_queue_size=%u", net->driver.virtio.rx_queue_size);
     }
 
-    if (usingVirtio && mtu) {
-        if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_VIRTIO_NET_HOST_MTU)) {
-
-            virBufferAsprintf(&buf, ",host_mtu=%u", mtu);
-
-        } else {
-            /* log an error if mtu was requested specifically for this
-             * interface, otherwise, if it's just what was reported by
-             * the attached network, ignore it.
-             */
-            if (net->mtu) {
-                virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
-                               _("setting MTU is not supported with "
-                                 "this QEMU binary"));
-                goto error;
-            }
+    if (usingVirtio && net->mtu) {
+        if (!virQEMUCapsGet(qemuCaps, QEMU_CAPS_VIRTIO_NET_HOST_MTU)) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                           _("setting MTU is not supported with this QEMU binary"));
+            goto error;
         }
+        virBufferAsprintf(&buf, ",host_mtu=%u", net->mtu);
     }
 
     if (vlan == -1)
@@ -8320,7 +8309,7 @@ qemuBuildVhostuserCommandLine(virQEMUDriverPtr driver,
     VIR_FREE(netdev);
 
     if (!(nic = qemuBuildNicDevStr(def, net, -1, bootindex,
-                                   queues, qemuCaps, net->mtu))) {
+                                   queues, qemuCaps))) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
                        "%s", _("Error generating NIC -device string"));
         goto error;
@@ -8366,7 +8355,6 @@ qemuBuildInterfaceCommandLine(virQEMUDriverPtr driver,
     virDomainNetType actualType = virDomainNetGetActualType(net);
     virNetDevBandwidthPtr actualBandwidth;
     size_t i;
-    unsigned int mtu = net->mtu;
 
 
     if (!bootindex)
@@ -8421,7 +8409,7 @@ qemuBuildInterfaceCommandLine(virQEMUDriverPtr driver,
         memset(tapfd, -1, tapfdSize * sizeof(tapfd[0]));
 
         if (qemuInterfaceBridgeConnect(def, driver, net,
-                                       tapfd, &tapfdSize, &mtu) < 0)
+                                       tapfd, &tapfdSize) < 0)
             goto cleanup;
         break;
 
@@ -8601,7 +8589,7 @@ qemuBuildInterfaceCommandLine(virQEMUDriverPtr driver,
     }
     if (qemuDomainSupportsNicdev(def, net)) {
         if (!(nic = qemuBuildNicDevStr(def, net, vlan, bootindex,
-                                       vhostfdSize, qemuCaps, mtu)))
+                                       vhostfdSize, qemuCaps)))
             goto cleanup;
         virCommandAddArgList(cmd, "-device", nic, NULL);
     } else {

src/qemu/qemu_command.h

@@ -101,8 +101,7 @@ char *qemuBuildNicDevStr(virDomainDefPtr def,
                          int vlan,
                          unsigned int bootindex,
                          size_t vhostfdSize,
-                         virQEMUCapsPtr qemuCaps,
-                         unsigned int mtu);
+                         virQEMUCapsPtr qemuCaps);
 
 char *qemuDeviceDriveHostAlias(virDomainDiskDefPtr disk);
 

src/qemu/qemu_hotplug.c

@@ -968,7 +968,6 @@ qemuDomainAttachNetDevice(virQEMUDriverPtr driver,
     bool charDevPlugged = false;
     bool netdevPlugged = false;
     bool hostPlugged = false;
-    unsigned int mtu = net->mtu;
 
     /* preallocate new slot for device */
     if (VIR_REALLOC_N(vm->def->nets, vm->def->nnets + 1) < 0)
@@ -1025,7 +1024,7 @@ qemuDomainAttachNetDevice(virQEMUDriverPtr driver,
             goto cleanup;
         memset(vhostfd, -1, sizeof(*vhostfd) * vhostfdSize);
         if (qemuInterfaceBridgeConnect(vm->def, driver, net,
-                                       tapfd, &tapfdSize, &mtu) < 0)
+                                       tapfd, &tapfdSize) < 0)
             goto cleanup;
         iface_connected = true;
         if (qemuInterfaceOpenVhostNet(vm->def, net, priv->qemuCaps,
@@ -1239,7 +1238,7 @@ qemuDomainAttachNetDevice(virQEMUDriverPtr driver,
         VIR_FORCE_CLOSE(vhostfd[i]);
 
     if (!(nicstr = qemuBuildNicDevStr(vm->def, net, vlan, 0,
-                                      queueSize, priv->qemuCaps, mtu)))
+                                      queueSize, priv->qemuCaps)))
         goto try_remove;
 
     qemuDomainObjEnterMonitor(driver, vm);

src/qemu/qemu_interface.c

@@ -503,8 +503,7 @@ qemuInterfaceBridgeConnect(virDomainDefPtr def,
                            virQEMUDriverPtr driver,
                            virDomainNetDefPtr net,
                            int *tapfd,
-                           size_t *tapfdSize,
-                           unsigned int *mtu)
+                           size_t *tapfdSize)
 {
     const char *brname;
     int ret = -1;
@@ -545,7 +544,7 @@ qemuInterfaceBridgeConnect(virDomainDefPtr def,
                                            def->uuid, tunpath, tapfd, *tapfdSize,
                                            virDomainNetGetActualVirtPortProfile(net),
                                            virDomainNetGetActualVlan(net),
-                                           net->coalesce, net->mtu, mtu,
+                                           net->coalesce, 0, NULL,
                                            tap_create_flags) < 0) {
             virDomainAuditNetDevice(def, net, tunpath, false);
             goto cleanup;

src/qemu/qemu_interface.h

@@ -51,8 +51,7 @@ int qemuInterfaceBridgeConnect(virDomainDefPtr def,
                                virQEMUDriverPtr driver,
                                virDomainNetDefPtr net,
                                int *tapfd,
-                               size_t *tapfdSize,
-                               unsigned int *mtu)
+                               size_t *tapfdSize)
     ATTRIBUTE_NONNULL(2);
 
 int qemuInterfaceOpenVhostNet(virDomainDefPtr def,