[Pkg-xen-devel] Bug#1067151: xen-utils-common: vif-openvswitch ignores MTU

[Hans van Kranenburg]

Hi Aleksi,

Thanks for the report. I actually ran into the same situation recently,
wanting to set up a PPPoE connection from within a Xen domU, also using
openvswitch as bridge.

On 19/03/2024 12:21, Aleksi Suhonen wrote:
> Package: src:xen
> Version: 4.17.3+10-g091466ba55-1~deb12u1
> Severity: wishlist
> 
> I wasn't sure if this script comes from Debian or Xen or somewhere else, 
> so I thought it safest to report it here.

These scripts/vif-* files are located in tools/hotplug/Linux in the Xen
source tree, we ship them as such in the Debian package. So, yes,
changes to them should first go upstream. However, it's perfectly fine
to have a discussion here, so we can figure out what the right changes
should be.

> /etc/xen/scripts/vif-bridge handles MTU settings in the vif, but the 
> otherwise similar /etc/xen/scripts/vif-openvswitch does not. I added it 
> in, here's the diff-c and the full fixed file is also attached.
> 
> *** vif-openvswitch.orig        2024-03-19 11:53:13.000000000 +0200
> --- vif-openvswitch     2024-03-19 11:56:17.000000000 +0200
> ***************
> *** 89,94 ****
> --- 89,95 ----
>        add|online)
>            check_tools
>            setup_virtual_bridge_port $dev
> +         set_mtu "$bridge" "$dev" "$type_if"
>            add_to_openvswitch $dev
>            ;;

Ah, interesting. I had some difficulties getting it to work back then.
But, when putting the set_mtu line back like this, it also gives me the
desired outcome now!

My use case is about setting up a PPPoE connection from a Xen domU over
vlan 6. I want an mtu of 1500 for the traffic inside the PPPoE
connection, so I need mtu 1508 for the connection between the PPPoE
client in the domU -> openvswitch in the dom0 -> physical interface ->
switchports -> ISP NTU device.

For some reason I had troubles to get the vifX.Y interface, as seen
inside dom0 set to mtu 1508. It seemed not to have any effect (using ip
link set mtu <M> dev <D>), or, openvswitch kept resetting it back to
1500 all the time. When I would use ovs-vsctl set interface <D>
mtu_request=<M> instead, it actually sticked. That's what I remember.

I just did some more testing, and I cannot really reproduce that
situation... :| I can also just use ip link in the dom0 now.

Interesting, but good, since it would mean that we can indeed just
(re)use that set_mtu function! :) I'm still curious what the problem was
when I tried earlier... Maybe anyone else reading this knows more?

Are you familiar with the process of sending patches upstream? Otherwise
we (Debian Xen team) can assist with that.

Regards,
Hans