[vfio-users] Nested VFIO with QEMU

[Alex Williamson]

On Fri, 18 Oct 2019 05:48:49 +0000
"Boeuf, Sebastien" <sebastien.boeuf at intel.com> wrote:

> Hi folks,
> 
> I have been recently working with VFIO, and particularly trying to
> achieve device passthrough through multiple layers of virtualization.
> 
> I wanted to assess QEMU's performances with nested VFIO, using the
> emulated Intel IOMMU device. Unfortunately, I cannot make any of my
> physical device work when I pass them through, attached to the emulated
> Intel IOMMU. Using regular VFIO works properly, but as soon as I enable
> the virtual IOMMU, the driver fails to probe (I tried on two different
> machines with different types of NIC).
> 
> So I was wondering if someone was aware of any issue with using both
> VFIO and virtual Intel IOMMU with QEMU? I'm sure I might be missing
> something obvious but I couldn't find it so far.

It's not something I test regularly, but I'm under the impression that
nested device assignment does work.  When you say the driver fails to
probe, which driver is that, the endpoint driver in the L2 guest or
vfio-pci in the L1 guest?  Perhaps share your XML or command line?
Thanks,

Alex