xennet0: rx no buf

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

I'm finding that my NetBSD 7 web-server VMs are often dropping off 
the network with "xennet0: rx no buf" errors, even under only moderate 
load. When this happens, the machines usually end up unresponsive and need 
to be hard-rebooted.

When this has been mentioned previously, there was an explanation of 
the cause, but no suggestions of how the problem may be mitigated. I'm 
not sure the explanation was correct either as the error is "rx no buf", 
not "rx no mbuf", i.e.:

https://nxr.netbsd.org/xref/src/sys/arch/xen/xen/if_xennet_xenbus.c#1079
not
https://nxr.netbsd.org/xref/src/sys/arch/xen/xen/if_xennet_xenbus.c#1068

What can be done to improve the situation?

--
Stephen