Network packet size over MTU in guests

[Fabrice Toppi <fabrice.toppi@openweb.be>]

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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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