[Xen-users] jumbo frames on PV or PVH

[pierre-philipp braun <pbraun@xxxxxxxxxxxx>]

Hello,

I am using,

xen & xen-tools 4.11-rc6
dom0 linux kernel 4.17.3
domU linux kernel 4.17.3
domU netbsd kernel 8.0

I enabled jumbo frames on the dom0 network interface and bridge.  When creating 
a PV guest, the vif shows up with the same MTU as the bridge.  The xen-tools 
network scripts are taking care of that.  However, onto the guest systems, I 
cannot set an MTU higher than 1500.

The guest configurations I am currently using,

kernel = "/data/kernels/vmlinuz"
root = "/dev/xvda1 ro console=hvc0"
name = "student1"
memory = "256"
vcpus = "2"
disk = ['tap:tapdisk:aio:/data/guests/student1/student1.reiser4,xvda1,w']
vif = [ 'bridge=xenbr0, vifname=student1.0' ]

kernel = "/data/kernels/netbsd8/netbsd-XEN3_DOMU.gz"
root = "xbd0a"
memory = 256
name = "netbsdffs"
vcpus = 2
disk = ['tap:tapdisk:aio:/data/guests/netbsdffs/netbsdffs.ffs,xvda,w']
vif = [ 'vifname=netbsdffs.0, bridge=pubbr0',
        'vifname=netbsdffs.1, bridge=xenbr0' ]

Trying to enable jumbo on a linux guest,

# ifconfig eth0 mtu 9000
SIOCSIFMTU: Invalid argument

so I look at the driver used and it seems to be e1000,

# dmesg | grep -i net
[reformatted]
e100: Intel(R) PRO/100 Network Driver, 3.5.24-k2-NAPI
e1000: Intel(R) PRO/1000 Network Driver - version 7.3.21-k8-NAPI
e1000e: Intel(R) PRO/1000 Network Driver - 3.2.6-k
xen_netfront: Initialising Xen virtual ethernet driver

same for netbsd/xen guest.  First, the ec capability `JUMBO_MTU` is missing,

# ifconfig xennet1
xennet1: flags=0x8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu 
1500
        capabilities=2800<TCP4CSUM_Tx,UDP4CSUM_Tx>
        enabled=0
        ec_capabilities=1<VLAN_MTU>
        ec_enabled=0
        address: 00:16:3e:01:14:93
        inet 10.1.1.99/24 broadcast 10.1.1.255 flags 0x0
        inet6 fe80::216:3eff:fe01:1493%xennet1/64 flags 0x0 scopeid 0x2

second, I got the same error as with the linux guest,

# ifconfig xennet1 mtu 1501
ifconfig: SIOCSIFMTU: Invalid argument

the driver is,

dmesg | grep -i net
[reformatted]
xennet1 at xenbus0 id 1: Xen Virtual Network Interface

Why do I see e1000 mentioned on the linux domU dmesg?  That Intel network 
controller does not allow jumbo frames in the real world.

Does it mean I cannot enable jumbo frames on PV guests?  Is it the backend or 
the frontend driver that is not allowing it?

Should I switch to PVH or even PVHVM?

Should I consider vif `type=ioemu` [1] and/or `model=virtio-net`?  Is that the 
only solution?  PVH seems to discard those settings just like PV.  Would that 
work with PVHVM?

Is virtio-net (or virtio-net-pci?) available nowadays?  Will I have to use 
`type=ioemu` for virtio-* to work?

I do not need to patch and recompile everything right [2]?

[1]: 
https://xenbits.xen.org/docs/4.11-testing/man/xl-network-configuration.5.html
[2]: https://wiki.xenproject.org/wiki/Virtio_On_Xen

I need to find out what direction to take.  Some hints will be welcome.
-- 
pierre-philipp

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