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I'm running several old 32-bit SLES 9 instances in libvirt/qemu virtual machines. Around 6 months ago after a kernel update (from 4.3.3) some of these virtual machines failed to get the network working. With a static IP address the network interface is up, but no traffic goes out or in.
After some investigation I figured out that this affected only virtual machines with more than one CPU. If I changed the number of CPUs to 1, network started working as expected. Tried also a fresh SLES 9 install on a new virtual machine to ensure that this was not up to the configuration in my old virtual machines, but got the same result. If the same virtual machines are booted with the Arch linux iso, networking works fine.
If I start up a fresh SLES 9 install in a virtual machine with 2 CPUs configured with a dynamic IP address, I can see that on the host machine the virtual machine requests for an IP address and is offered one, but on the guest machine no IP address is received. Changing the number of CPUs to 1 solves the issue immediately.
So far I have tried different combinations of number of CPUs, memory sizes, rtl8139 and e1000 network devices (cannot use virtio due to lack of drivers in SLES9). Anything else that could be tried except of going back to the 4.3.3 kernel or replacing SLES 9 with something more modern?
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A bit late I suppose, but setting the network card's IRQ affinity to CPU 0 solved it for me.
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