[PATCH] qemu: command: Don't attempt to set backend MTU for networks which don't use host backend directly
From: Peter Krempa <pkrempa@redhat.com> 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> --- src/qemu/qemu_command.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) 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; -- 2.49.0
On a Monday in 2025, Peter Krempa via Devel wrote:
From: Peter Krempa <pkrempa@redhat.com>
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> --- src/qemu/qemu_command.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
Reviewed-by: Ján Tomko <jtomko@redhat.com> Jano
On 6/2/25 11:00 AM, Peter Krempa via Devel wrote:
From: Peter Krempa <pkrempa@redhat.com>
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.
It's disturbing that this points out I had added the necessary code to add "--mtu" to the passt commandline when <mtu size='blah'/> was specified, but apparently had never tested it (because it would have failed) :-/ Anyway, on the upside, according to Stefano it's always best to leave mtu unspecified for passt interfaces, because it then defaults to some gigantic number that gives the best performance :-) (Oh, BTW I built with and without your patch, with the expected results :-)
participants (3)
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Ján Tomko -
Laine Stump -
Peter Krempa