qemu: command: Don't attempt to set backend MTU for networks which don't use host...

Attempting to set MTU for network types which don't actually use the
network device on the host results in a failure. The 'mtu' property is
also used e.g. for the 'host_mtu' property of e.g. 'virtio-net-pci'
which is applied even in vhost-user mode.

Use the existing switch which selects devices without a network device
backend on the host side and skip setting the MTU.

Tested by running 'passt' in vhost-user mode manually:

 passt -f --vhost-user -s /tmp/vh.sock

and the following XML:

 <interface type="vhostuser">
   <mac address="52:54:00:3d:91:97"/>
   <source type="unix" path="/tmp/vh.sock" mode="client"/>
   <model type="virtio"/>
   <mtu size="9999"/>
   <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x0"/>
 </interface>

The OS in the guest reports MTU 9999.

Closes: https://gitlab.com/libvirt/libvirt/-/issues/717
Closes: https://gitlab.com/libvirt/libvirt/-/issues/192
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index fec48edfc1daa410822dd6d4d09c702159fd64c1..edafe1588c751095d2dd4a4367a870969a58218d 100644 (file)
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -8713,6 +8713,7 @@ qemuBuildInterfaceCommandLine(virQEMUDriver *driver,
     bool requireNicdev = false;
     g_autoptr(virJSONValue) hostnetprops = NULL;
     qemuDomainNetworkPrivate *netpriv = QEMU_DOMAIN_NETWORK_PRIVATE(net);
+    bool setBackendMTU = true;
     GSList *n;
 
     if (qemuDomainValidateActualNetDef(net, qemuCaps) < 0)
@@ -8802,6 +8803,7 @@ qemuBuildInterfaceCommandLine(virQEMUDriver *driver,
     case VIR_DOMAIN_NET_TYPE_NULL:
     case VIR_DOMAIN_NET_TYPE_VDS:
     case VIR_DOMAIN_NET_TYPE_LAST:
+        setBackendMTU = false;
        /* These types don't use a network device on the host, but
         * instead use some other type of connection to the emulated
         * device in the qemu process.
@@ -8842,7 +8844,7 @@ qemuBuildInterfaceCommandLine(virQEMUDriver *driver,
         }
     }
 
-    if (net->mtu && net->managed_tap != VIR_TRISTATE_BOOL_NO &&
+    if (net->mtu && setBackendMTU && net->managed_tap != VIR_TRISTATE_BOOL_NO &&
         virNetDevSetMTU(net->ifname, net->mtu) < 0)
         goto cleanup;