[CentOS-virt] PCI Passthrough not working

Mon May 16 12:44:18 UTC 2016
George Dunlap <dunlapg at umich.edu>

On Mon, May 16, 2016 at 12:00 PM, Francis Greaves <francis at choughs.net> wrote:
> Dear George please find attached the three files as requested.
> I have used
> iommu=soft
> in the grub command line for the kernel in the domU as explained before.
> many thanks

(Please reply in-line, like this, rather than top-posting.)

Thanks -- as I suspected, your USB device has RMRRs, which are what's
causing the problem.

In this case, your USB controller's RMRRs are shared with another
device.  Theoretically, having two devices with the same RMRRs
assigned to different domains could be a security issue, so Xen by
default refuses to do it.

The options are:
1. Figure out what the other device is and assign them both to the guest
2. Tell Xen that you don't mind the sharing.

You should only do #2 if you're not using Xen for isolation -- i.e.,
if you trust the software in that VM not to attack dom0.

I *think* you should be able to do #2 by adding 'rdm_policy=relaxed'
to your pci stanza; i.e., it should look like this:


Let me know if that works for you.