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?). Regards, Ingo Krabbe
