Network packet size over MTU in guests

Network packet size over MTU in guests

[Fabrice Toppi]

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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 ?

Thanks.

FT

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Re: Network packet size over MTU in guests

[Michael Tokarev]

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