how to network jumbo frames to a kvm guest

Getting a jumbo frame to a KVM guest is not something which works out of the box, but can be configured to work quite easily.

One way get jumbo frames to a KVM guest is to directly pass the guest a physical NIC through PCI Passthrough, or a Virtual Function through SR-IOV.

Another way is to use the traditional libvirt bridge. Create a bridge on the host, and bridge the guest's NICs into this. The reason this doesn't work by default is all the interfaces are created with the default Ethernet MTU of 1500 bytes.

You can set MTU of the physical NIC, the bridge, and the guest interface with the network-scripts, but you cannot automatically set the MTU of the tap interface with the network-scripts. The tap interface is how the virtual network interface connects to the "real world". You'll see these tap devices with names like "vnet0" in your bridge.

So how do we do this?

First, some requirements:

• A physical interface which supports jumbo frames.
  The Linux bridge code will only allow the bridge to change to the smallest MTU of all its interfaces.
• A bridge with the larger required MTU
  Technically a jumbo frame covers MTU from 1501 to 9000, but almost everyone just wants to set 9000
  
• virtio-net or Intel e1000 network interface inside the guest
  The real-life Realtek 8139 does not support an MTU of 9000, therefore there is no driver support for large MTU in the emulated version either.

Our first step is to make all tap interfaces on the host have an MTU of 9000. We do this by adding a udev rule to the top of the network device creation as follows:

/etc/udev/rules.d/70-persistent-net.rules (on host)

SUBSYSTEM=="net", ACTION=="add", KERNEL=="vnet*", ATTR{mtu}="9000"

Then set your physical interface MTU to 9000:

/etc/sysconfig/network-scripts/ifcfg-eth0 (on host):

DEVICE=eth0
HWADDR=[physical MAC address]
TYPE=Ethernet
ONBOOT=yes
BRIDGE=br0
MTU=9000

And do the same inside the bridge:

/etc/sysconfig/network-scripts/ifcfg-br0 (on host)

DEVICE=br0
TYPE=Bridge
ONBOOT=yes
DELAY=0
MTU=9000

Now inside your guest, set your virtio or e1000 interface to also have the larger MTU:

/etc/sysconfig/network-scripts/ifcfg-eth0 (in guest)

DEVICE=eth0
HWADDR=[guest MAC address]
TYPE=Ethernet
ONBOOT=yes
MTU=9000

You'll need to perform a "service network restart" on the host, or at least bring the bridge and eth interfaces down and up. Start your guest and confirm all interfaces have been made with the correct MTU:

# ip link
2: eth0: mtu 9000 qdisc pfifo_fast state UP qlen 1000
    link/ether 01:23:45:67:89:0A brd ff:ff:ff:ff:ff:ff
3: br0: mtu 9000 qdisc noqueue state UNKNOWN
    link/ether 01:23:45:67:89:0A brd ff:ff:ff:ff:ff:ff
4: vnet0: mtu 9000 qdisc pfifo_fast state UNKNOWN qlen 500
    link/ether fe:54:00:01:23:45 brd ff:ff:ff:ff:ff:ff

This should be enough to send a "Do Not Fragment" 9000 byte frame over the network. Let's test with a ping:

# ping -c 4 -s 8972 -M do 172.16.0.2
PING 172.16.0.2 (172.16.0.2) 8972(9000) bytes of data.
8980 bytes from 172.16.0.2: icmp_seq=1 ttl=64 time=1.27 ms
8980 bytes from 172.16.0.2: icmp_seq=2 ttl=64 time=0.284 ms
8980 bytes from 172.16.0.2: icmp_seq=3 ttl=64 time=0.202 ms
8980 bytes from 172.16.0.2: icmp_seq=4 ttl=64 time=0.260 ms

Success!