Re: Network packet size over MTU in guests

[Michael Tokarev <mjt@tls.msk.ru>]

Fabrice Toppi wrote:
> Hi,
> 
> We use KVM 75 on Gentoo hosts with kernel 2.6.27.8 with a variety of 
> guests (Windows XP, 2000, 2003 & several Linux distributions) in a 
> production setup.
> The hosts are Intel Xeon based. We use bridged networking with E1000 
> model inside of guests.
> 
> Recently, we began to have network problem (slowdowns, lags, timeouts) 
> on several guests. Inside those guests, we saw with tcpdump that there 
> were packets with a size well over MTU. On the host, tcpdump on the 
> guest tap did show packets of 1500 bytes and that's all.
> For some of the guests (Debian based), changing the NIC model to rtl8139 
> instead of e1000 "fixed" the problem, but for others (Gentoo kernel 
> 2.6.27.19) it didn't.
> 
> Would it be a glitch in the KVM code or a driver problem ? Does this 
> ring a bell to anyone ?

I'm not sure anymore but I think it's very similar to what I had here with
pre-78 (approx again, since it was quite some time ago) versions of kvm
compiled with older kernel headers (without appropriate if_tun.h definitions).

I'd say try to reproduce it with 0.11.0 kvm userspace compiled against
2.6.27+ kernel headers.  But yet again: kvm-75 is quite old by now, so
it's difficult to remember all the details.

/mjt