From: Peter Krempa <pkrempa@redhat.com>
Date: Mon, 2 Jun 2025 14:55:16 +0000 (+0200)
Subject: qemu: command: Don't attempt to set backend MTU for networks which don't use host... 
X-Git-Tag: v11.5.0-rc1~85
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=c61d6d060667020521b813af152560d8da8d63ab;p=thirdparty%2Flibvirt.git

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

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 fec48edfc1..edafe1588c 100644
--- 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;