[pve-devel] [PATCH qemu-server v3 1/1] net: automatically set host_mtu to bridge mtu for virtio netdevs
Stefan Hanreich
s.hanreich at proxmox.com
Thu Jul 17 19:50:10 CEST 2025
When creating a new network device from the UI and leaving the MTU
field empty, it defaults to 1500. This is inconvenient in cases where
the MTU of the bridge is not 1500 and lead to some confusion of users
[1]. Containers already inherit the bridge MTU when the field is left
empty, so align the behavior of VMs to be more in line with the more
convenient behavior of containers.
The common case where this was encountered was with creating network
devices on SDN VXLAN vnets. There the default MTU for bridges is 1450,
since VXLAN adds some overhead and we automatically subtract that
overhead from the default bridge MTU (1500) if no MTU is explicitly
set in the zone configuration. Before that users always had to
explicitly set the MTU to 1450 or 1 for every network device created,
which is error-prone.
[1] https://forum.proxmox.com/threads/bug-vxlan-and-mtu.161412
Signed-off-by: Stefan Hanreich <s.hanreich at proxmox.com>
---
src/PVE/QemuServer.pm | 30 ++++++++++++++------------
src/PVE/QemuServer/Network.pm | 3 ++-
src/test/cfg2cmd/netdev_vxlan.conf | 7 ++++++
src/test/cfg2cmd/netdev_vxlan.conf.cmd | 28 ++++++++++++++++++++++++
src/test/run_config2command_tests.pl | 6 ++++++
5 files changed, 59 insertions(+), 15 deletions(-)
create mode 100644 src/test/cfg2cmd/netdev_vxlan.conf
create mode 100644 src/test/cfg2cmd/netdev_vxlan.conf.cmd
diff --git a/src/PVE/QemuServer.pm b/src/PVE/QemuServer.pm
index ec554c4a..d7398648 100644
--- a/src/PVE/QemuServer.pm
+++ b/src/PVE/QemuServer.pm
@@ -1452,21 +1452,23 @@ sub print_netdevice_full {
$tmpstr .= ",bootindex=$net->{bootindex}" if $net->{bootindex};
- if (my $mtu = $net->{mtu}) {
- if ($net->{model} eq 'virtio' && $net->{bridge}) {
- my $bridge_mtu = PVE::Network::read_bridge_mtu($net->{bridge});
- if ($mtu == 1) {
- $mtu = $bridge_mtu;
- } elsif ($mtu < 576) {
- die "netdev $netid: MTU '$mtu' is smaller than the IP minimum MTU '576'\n";
- } elsif ($mtu > $bridge_mtu) {
- die "netdev $netid: MTU '$mtu' is bigger than the bridge MTU '$bridge_mtu'\n";
- }
- $tmpstr .= ",host_mtu=$mtu";
- } else {
- warn
- "WARN: netdev $netid: ignoring MTU '$mtu', not using VirtIO or no bridge configured.\n";
+ my $mtu = $net->{mtu};
+
+ if ($net->{model} eq 'virtio' && $net->{bridge}) {
+ my $bridge_mtu = PVE::Network::read_bridge_mtu($net->{bridge});
+
+ if (!defined($mtu) || $mtu == 1) {
+ $mtu = $bridge_mtu;
+ } elsif ($mtu < 576) {
+ die "netdev $netid: MTU '$mtu' is smaller than the IP minimum MTU '576'\n";
+ } elsif ($mtu > $bridge_mtu) {
+ die "netdev $netid: MTU '$mtu' is bigger than the bridge MTU '$bridge_mtu'\n";
}
+
+ $tmpstr .= ",host_mtu=$mtu" if $mtu != 1500;
+ } elsif (defined($mtu)) {
+ warn
+ "WARN: netdev $netid: ignoring MTU '$mtu', not using VirtIO or no bridge configured.\n";
}
if ($use_old_bios_files) {
diff --git a/src/PVE/QemuServer/Network.pm b/src/PVE/QemuServer/Network.pm
index 9ca31435..56df83fb 100644
--- a/src/PVE/QemuServer/Network.pm
+++ b/src/PVE/QemuServer/Network.pm
@@ -110,7 +110,8 @@ my $net_fmt = {
type => 'integer',
minimum => 1,
maximum => 65520,
- description => "Force MTU, for VirtIO only. Set to '1' to use the bridge MTU",
+ description =>
+ "Force MTU of network device (VirtIO only). Setting to '1' or empty will use the bridge MTU",
optional => 1,
},
};
diff --git a/src/test/cfg2cmd/netdev_vxlan.conf b/src/test/cfg2cmd/netdev_vxlan.conf
new file mode 100644
index 00000000..af9e31c7
--- /dev/null
+++ b/src/test/cfg2cmd/netdev_vxlan.conf
@@ -0,0 +1,7 @@
+# TEST: Test inheriting the MTU from a bridge with MTU != 1500
+bootdisk: scsi0
+cores: 3
+memory: 768
+name: netdev
+net0: virtio=A2:C0:43:77:08:A0,bridge=vxlan_bridge
+ostype: l26
diff --git a/src/test/cfg2cmd/netdev_vxlan.conf.cmd b/src/test/cfg2cmd/netdev_vxlan.conf.cmd
new file mode 100644
index 00000000..a2f3579d
--- /dev/null
+++ b/src/test/cfg2cmd/netdev_vxlan.conf.cmd
@@ -0,0 +1,28 @@
+/usr/bin/kvm \
+ -id 8006 \
+ -name 'netdev,debug-threads=on' \
+ -no-shutdown \
+ -chardev 'socket,id=qmp,path=/var/run/qemu-server/8006.qmp,server=on,wait=off' \
+ -mon 'chardev=qmp,mode=control' \
+ -chardev 'socket,id=qmp-event,path=/var/run/qmeventd.sock,reconnect-ms=5000' \
+ -mon 'chardev=qmp-event,mode=control' \
+ -pidfile /var/run/qemu-server/8006.pid \
+ -daemonize \
+ -smp '3,sockets=1,cores=3,maxcpus=3' \
+ -nodefaults \
+ -boot 'menu=on,strict=on,reboot-timeout=1000,splash=/usr/share/qemu-server/bootsplash.jpg' \
+ -vnc 'unix:/var/run/qemu-server/8006.vnc,password=on' \
+ -cpu kvm64,enforce,+kvm_pv_eoi,+kvm_pv_unhalt,+lahf_lm,+sep \
+ -m 768 \
+ -global 'PIIX4_PM.disable_s3=1' \
+ -global 'PIIX4_PM.disable_s4=1' \
+ -device 'pci-bridge,id=pci.1,chassis_nr=1,bus=pci.0,addr=0x1e' \
+ -device 'pci-bridge,id=pci.2,chassis_nr=2,bus=pci.0,addr=0x1f' \
+ -device 'piix3-usb-uhci,id=uhci,bus=pci.0,addr=0x1.0x2' \
+ -device 'usb-tablet,id=tablet,bus=uhci.0,port=1' \
+ -device 'VGA,id=vga,bus=pci.0,addr=0x2' \
+ -device 'virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3,free-page-reporting=on' \
+ -iscsi 'initiator-name=iqn.1993-08.org.debian:01:aabbccddeeff' \
+ -netdev 'type=tap,id=net0,ifname=tap8006i0,script=/usr/libexec/qemu-server/pve-bridge,downscript=/usr/libexec/qemu-server/pve-bridgedown,vhost=on' \
+ -device 'virtio-net-pci,mac=A2:C0:43:77:08:A0,netdev=net0,bus=pci.0,addr=0x12,id=net0,rx_queue_size=1024,tx_queue_size=256,bootindex=300,host_mtu=1450' \
+ -machine 'type=pc+pve0'
diff --git a/src/test/run_config2command_tests.pl b/src/test/run_config2command_tests.pl
index 16a56987..ab781e00 100755
--- a/src/test/run_config2command_tests.pl
+++ b/src/test/run_config2command_tests.pl
@@ -439,6 +439,12 @@ my $pve_common_network;
$pve_common_network = Test::MockModule->new('PVE::Network');
$pve_common_network->mock(
read_bridge_mtu => sub {
+ my ($bridge_name) = @_;
+
+ if ($bridge_name eq 'vxlan_bridge') {
+ return 1450;
+ }
+
return 1500;
},
);
--
2.39.5
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