[pve-devel] [PATCH qemu-server 1/1] net: pass host_mtu parameter when mtu is unset in netdev config

* [pve-devel] [PATCH qemu-server 1/1] net: pass host_mtu parameter when mtu is unset in netdev config
@ 2025-04-17  9:59 Stefan Hanreich
  2025-04-17 10:50 ` [pve-devel] superseded: " Stefan Hanreich
  0 siblings, 1 reply; 2+ messages in thread
From: Stefan Hanreich @ 2025-04-17  9:59 UTC (permalink / raw)
  To: pve-devel

When creating a new network device from the UI and leaving the MTU
field empty, the field hints that this means that the MTU is inherited
from the bridge. Since leaving the field empty omits the key from the
network device configuration, we need to inherit from the bridge MTU
when this key is missing as well, instead of only inheriting from the
bridge when the field is explicitly set to 1.

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.

I decided to fix this here, instead of in the UI, since the fix then
also works for existing network devices where MTU is unset. This means
that users would not have to re-apply the setting from the UI. It also
seems like the sensible default to inherit the MTU value from the
bridge when this key doesn't exist instead of just not setting
host_mtu at all.

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
---
 PVE/QemuServer.pm                  | 28 +++++++++++++++-------------
 test/cfg2cmd/netdev_vxlan.conf     |  8 ++++++++
 test/cfg2cmd/netdev_vxlan.conf.cmd | 26 ++++++++++++++++++++++++++
 test/run_config2command_tests.pl   |  6 ++++++
 4 files changed, 55 insertions(+), 13 deletions(-)
 create mode 100644 test/cfg2cmd/netdev_vxlan.conf
 create mode 100644 test/cfg2cmd/netdev_vxlan.conf.cmd

diff --git a/PVE/QemuServer.pm b/PVE/QemuServer.pm
index ccdceedc..6a55d59f 100644
--- a/PVE/QemuServer.pm
+++ b/PVE/QemuServer.pm
@@ -1596,20 +1596,22 @@ 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/test/cfg2cmd/netdev_vxlan.conf b/test/cfg2cmd/netdev_vxlan.conf
new file mode 100644
index 00000000..222a66d4
--- /dev/null
+++ b/test/cfg2cmd/netdev_vxlan.conf
@@ -0,0 +1,8 @@
+# TEST: Simple test for netdev related stuff
+# QEMU_VERSION: 5.0
+bootdisk: scsi0
+cores: 3
+memory: 768
+name: netdev
+net0: virtio=A2:C0:43:77:08:A0,bridge=vxlan_bridge
+ostype: l26
diff --git a/test/cfg2cmd/netdev_vxlan.conf.cmd b/test/cfg2cmd/netdev_vxlan.conf.cmd
new file mode 100644
index 00000000..a9db44bf
--- /dev/null
+++ b/test/cfg2cmd/netdev_vxlan.conf.cmd
@@ -0,0 +1,26 @@
+/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=5' \
+  -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 \
+  -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' \
+  -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,bootindex=300,host_mtu=1450' \
+  -machine 'type=pc+pve0'
diff --git a/test/run_config2command_tests.pl b/test/run_config2command_tests.pl
index 209122c2..c2dfbd31 100755
--- a/test/run_config2command_tests.pl
+++ b/test/run_config2command_tests.pl
@@ -316,6 +316,12 @@ 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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