Issue with packet framing on xnb(4) connections and NetBSD domus

[Roger Pau Monné]

On Wed, Mar 17, 2021 at 01:23:46AM -0700, Brian Buhrow wrote:
> 	hello.  I've noticed that on my Xen server, running FreeBSD-12.2/Xen-4.14.0, I see a lot
> of messages like the following from my NetBSD-5 and NetBSD-current domu's: 
> xennet0: discarding oversize frame (len=1518)
> 
> They happen when network traffic is heavy and when traffic is coming from a physical switch in
> the same vlan as the domu's.  Since I don't think packets are  actually oversized on the wire,
> the Cisco switch managing this network does not report any giant packets, I think this is a
> software problem either on the NetBSD domu's, or the FreeBSD dom0.  Since I've been running
> these same NetBSD domu setups on a machine where NetBSD is the dom0, I think this is an issue
> with the xnb(4) or netback.c  driver.  Specifically, it looks like it concatinates multiple
> packets together before notifying the domu that there is traffic available.  Perhaps that is by
> design, since I cannot reproduce the issue on a FreeBSD-domu running on the same FreeBSD-dom0
> machine.  In either case, while things are usable, it creates a very noisy log on the
> NetBSD-domu machines.  Does the xnb(4) driver forward multiple packets to the domu front ends
> and set the length of the message to the sum of the lengths of all the packets it's forwarding,
> leaving it to the domu front end driver to separate the packets on reception?  Is there a
> parameter that can be set on a per-domu basis to say whether you want
> multiple-packets-per-transfer or not from the dom0?

I think this is an issue with FreeBSD netback as it doesn't properly
handle frontends that don't understand some of the features
implemented in the FreeBSD backend and are enabled unconditionally.

Could you paste the output of `xenstore-ls -fp` from dom0 when one of
those NetBSD domUs is running?

Thanks, Roger.