[pve-devel] [PATCH docs/manager/qemu-server v3 0/3] Make VirtIO network devices inherit MTU from bridge by default

[pve-devel] [PATCH docs/manager/qemu-server v3 0/3] Make VirtIO network devices inherit MTU from bridge by default

[Stefan Hanreich]

The current default behavior for VirtIO network devices is to default to 1500
MTU, unless otherwise specified. This is inconvenient in cases where the MTU is
not the default value (e.g. for VXLAN VNets or bridges with jumbo frames).
Containers already inherit the MTU of the bridge, if not set, so change the
behavior of VMs to be more in line with containers. This also makes using
non-standard MTUs more convenient and less error-prone since users do not have
to remember setting the MTU everytime they configure a network device on such a
brige.

Changes from v2:
* rebase on top of re-formatted code
* fix tests with new qemu version

Changes from v1:
* better document this behavior
* show better hints in the UI
* improve test case

qemu-server:

Stefan Hanreich (1):
  net: automatically set host_mtu to bridge mtu for virtio netdevs

 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


pve-manager:

Stefan Hanreich (1):
  qemu: network: adjust MTU emptyText to match new default behavior

 www/manager6/qemu/NetworkEdit.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)


pve-docs:

Stefan Hanreich (1):
  qm: document new default behavior for mtu setting

 qm.adoc | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)


Summary over all repositories:
  7 files changed, 63 insertions(+), 20 deletions(-)

-- 
Generated by git-murpp 0.8.0

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[pve-devel] [PATCH qemu-server v3 1/1] net: automatically set host_mtu to bridge mtu for virtio netdevs

[Stefan Hanreich]

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@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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Re: [pve-devel] [PATCH qemu-server v3 1/1] net: automatically set host_mtu to bridge mtu for virtio netdevs

[Thomas Lamprecht]

On Thu, 17 Jul 2025 19:50:10 +0200, Stefan Hanreich wrote:
> 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.
> 
> [...]

Applied, thanks!

[1/1] net: automatically set host_mtu to bridge mtu for virtio netdevs
      commit: 84a937c14ac36826b073543f64fe41dbd4a0d55e


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[pve-devel] [PATCH pve-manager v3 1/1] qemu: network: adjust MTU emptyText to match new default behavior

[Stefan Hanreich]

Leaving the MTU field unset now defaults to the bridge MTU, rather
than 1500. Reflect this change by indicating the new behavior in the
emptyText of the MTU field. While we're at it, add a gettext call so
it can be translated. I've taken the same text as from the container
dialogue, so it should already use existing translations.

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
---
 www/manager6/qemu/NetworkEdit.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/www/manager6/qemu/NetworkEdit.js b/www/manager6/qemu/NetworkEdit.js
index 05ba3e8d3..2ba13c40d 100644
--- a/www/manager6/qemu/NetworkEdit.js
+++ b/www/manager6/qemu/NetworkEdit.js
@@ -113,7 +113,7 @@ Ext.define('PVE.qemu.NetworkInputPanel', {
                     disabled: '{!isVirtio}',
                     value: '{mtu}',
                 },
-                emptyText: '1500 (1 = bridge MTU)',
+                emptyText: gettext('Same as bridge'),
                 minValue: 1,
                 maxValue: 65520,
                 allowBlank: true,
-- 
2.39.5


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Re: [pve-devel] [PATCH pve-manager v3 1/1] qemu: network: adjust MTU emptyText to match new default behavior

[Thomas Lamprecht]

On Thu, 17 Jul 2025 19:50:11 +0200, Stefan Hanreich wrote:
> Leaving the MTU field unset now defaults to the bridge MTU, rather
> than 1500. Reflect this change by indicating the new behavior in the
> emptyText of the MTU field. While we're at it, add a gettext call so
> it can be translated. I've taken the same text as from the container
> dialogue, so it should already use existing translations.
> 
> 
> [...]

Applied, thanks!

[1/1] qemu: network: adjust MTU emptyText to match new default behavior


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[pve-devel] [PATCH pve-docs v3 1/1] qm: document new default behavior for mtu setting

[Stefan Hanreich]

The default behavior changed from setting 1500 MTU to inheriting the
bridge MTU, if the MTU key for network devices is unset.

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
---
 qm.adoc | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/qm.adoc b/qm.adoc
index 393242a..0548c46 100644
--- a/qm.adoc
+++ b/qm.adoc
@@ -924,10 +924,9 @@ but not via the web UI.
 You can also skip adding a network device when creating a VM by selecting *No
 network device*.
 
-You can overwrite the *MTU* setting for each VM network device. The option
-`mtu=1` represents a special case, in which the MTU value will be inherited
-from the underlying bridge.
-This option is only available for *VirtIO* network devices.
+You can overwrite the *MTU* setting for each VM network device manually. Leaving
+the field empty or setting `mtu=1` will inherit the MTU from the underlying
+bridge. This option is only available for *VirtIO* network devices.
 
 .Multiqueue
 If you are using the VirtIO driver, you can optionally activate the
-- 
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Re: [pve-devel] [PATCH pve-docs v3 1/1] qm: document new default behavior for mtu setting

[Thomas Lamprecht]

On Thu, 17 Jul 2025 19:50:12 +0200, Stefan Hanreich wrote:
> The default behavior changed from setting 1500 MTU to inheriting the
> bridge MTU, if the MTU key for network devices is unset.
> 
> 

Applied, thanks!

[1/1] qm: document new default behavior for mtu setting


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