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[Qemu-devel] TCP Segementation Offloading

From: Ingo Krabbe
Subject: [Qemu-devel] TCP Segementation Offloading
Date: Sun, 1 May 2016 14:31:57 +0200

Good Mayday Qemu Developers,

today I tried to find a reference to a networking problem, that seems to be of 
quite general nature: TCP Segmentation Offloading (TSO) in virtual environments.

When I setup TAP network adapter for a virtual machine and put it into a host 
bridge, the known best practice is to manually set "tso off gso off" with 
ethtool, for the guest driver if I use a hardware emulation, such as e1000 
and/or "tso off gso off" for the host driver and/or for the bridge adapter, if 
I use the virtio driver, as otherwise you experience (sometimes?) performance 
problems or even lost packages.

I haven't found a complete analysis of the background of these problems, but 
there seem to be some effects on MTU based fragmentation and UDP checksums.

There is a tso related bug on launchpad, but the context of this bug is too 
narrow, for the generality of the problem.

Also it seems that there is a problem in LXC contexts too (I found such a 
reference, without detailed description in a Post about Xen setup).

My question now is: Is there a bug in the driver code and shouldn't this be 
documented somewhere in wiki.qemu.org? Where there developments about this 
topic in the past or is there any planned/ongoing work todo on the qemu drivers?

Most problem reports found relate to deprecated Centos6 qemu-kvm packages.

In our company we have similar or even worse problems with Centos7 hosts and 
guest machines.

I'm going to analyze these problems next week anyway and I woud be happy to 
share my observation with you. (Where can I register for the wiki, or whom 
should I sent my reports about this topic?).


Ingo Krabbe

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