[Qemu-devel] Virtual IOMMU + Virtio-net devices in a Windows VM doesn't work

[Jintack Lim]

Hi,

I'm running a Windows VM on top of KVM on x86, and one of virtio-net
device in the Windows VM doesn't seem to work. I provided virtual
IOMMU and two virtio-net devices to the VM: one bypassing the virtual
IOMMU and the other one behind the virtual IOMMU[1]. It turned out
that the virtio-net device behind virtual IOMMU didn't work while the
one bypassing the virtual IOMMU worked well. In a linux VM with the
same configuration, both of virtio-net device worked well.

I found that there is a subtle difference between virtio-net devices
bypassing and behind virtual IOMMU in a Linux VM. The lscpu command in
the Linux VM shows different device names for them; the first line is
for the bypassing one, and the second line is for the one behind the
virtual IOMMU

00:03.0 Ethernet controller: Red Hat, Inc Virtio network device
01:00.0 Ethernet controller: Red Hat, Inc Device 1041 (rev 01)

I wonder if this difference somehow caused the problem in the Windows
VM. I've installed the latest virtio drivers (0.1.149) from the fedora
project [2]

Any thoughts?

I'm using v4.15 Linux kernel as a host, and QEMU 2.11.0.

Thanks,
Jintack

[1] https://wiki.qemu.org/Features/VT-d
[2] 
https://docs.fedoraproject.org/quick-docs/en-US/creating-windows-virtual-machines-using-virtio-drivers.html