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

[Aleksi Suhonen]

Hi,

On 3/19/24 5:23 PM, Hans van Kranenburg wrote:
>> 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.

Fair.

>> +         set_mtu "$bridge" "$dev" "$type_if"

> 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!

I think the scripts were different back then. I wrote my own vif scripts 
as well, to get the MTU thing and a couple of other things right, 
because I couldn't figure out how to get the built in scripts to do it.

I was building a couple of new Xen hosts this winter, and noticed that 
the scripts looked much better now, and gave them a go, and only found 
the MTU issue and the IPv6 link locals that were bugging me.

When I grepped the script dir for mtu, I noticed the shared function in 
network tools that only vif-bridge was using, and tried that. I suspect 
it could be added into a lot of the other vif scripts as well...

> 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?

I wonder if some other component in ovs had the wrong MTU and that kept 
overriding the others... There's the uplink port, the virtual switch and 
the vif port. I think the virtual switch is picky that all the ports 
should be the same MTU.

Another issue I remember having like a decade ago was that some IXGBE 
card seemed to refuse to take new MTU settings after it was already in 
production.

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

Ah, no. Any help is much appreciated.

Best Regards,

-- 
	Aleksi Suhonen

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