This has been requested several times in #virt on IRC, and on libvirt-users, and a comment posted to https://bugzilla.redhat.com/1224348 reminded me that there's even an open BZ for it, so I finally sat down and wrote the patches for it. The idea of the patches is that an *empty* bridge device can't have its MTU directly set to anything higher than 1500, but once there are any devices on the bridge, its MTU is limited to the smallest of the MTUs of any attached devices. However, when the very first device is attached to the bridge, the bridge will *increase* its own MTU to match if needed. So the trick to setting a high MTU is to set the MTU of every device to the desired value before it is added (especially the first). libvirt already sets the MTU of every device to the MTU of the bridge, so all we need to do is make it so the first device is set from the network config's "mtu" attribute instead - the bridge will get the larger MTU, and all future devices added will get the higher MTU as well. This doesn't take care of setting the MTU on the guest side of the tap device; for now that must be done manually, but qemu has recently added a commandline parameter to send the desired MTU to the guest, and the virtio-net guest driver at least honors this. An upcoming patch to libvirt will automatically set this qemu commandline option so that no intervention will be required (beyond setting the MTU once in the network config). Laine Stump (3): util: add MTU arg to virNetDevTapCreateInBridgePort() conf: support mtu attribute in a network's <bridge> element network: honor mtu setting when creating network docs/formatnetwork.html.in | 8 +++++++- docs/news.xml | 6 +++++- docs/schemas/network.rng | 6 ++++++ src/bhyve/bhyve_command.c | 2 +- src/conf/network_conf.c | 20 ++++++++++++++++++-- src/conf/network_conf.h | 1 + src/network/bridge_driver.c | 1 + src/qemu/qemu_interface.c | 2 +- src/uml/uml_conf.c | 2 +- src/util/virnetdevtap.c | 20 +++++++++++++++----- src/util/virnetdevtap.h | 1 + tests/bhyvexml2argvmock.c | 1 + tests/networkxml2xmlin/set-mtu.xml | 11 +++++++++++ tests/networkxml2xmlout/set-mtu.xml | 11 +++++++++++ tests/networkxml2xmltest.c | 1 + 15 files changed, 81 insertions(+), 12 deletions(-) create mode 100644 tests/networkxml2xmlin/set-mtu.xml create mode 100644 tests/networkxml2xmlout/set-mtu.xml -- 2.9.3 -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
