I seem to have it working; I'll outline the steps in case others
run into this. Nevertheless, I'd appreciate an 'authoritative answer'
since I was 'fishing blindly'. [More see inline]

Am Freitag, 2. März 2018 09:34:18 UTC+1 schrieb sbor...@gmail.com:
> Dear Qubes community,
> 
> after using 3.1 and 3.2 in production on my primary laptop 
> (Lenovo X220), and having used that machine to test Qubes since R2,
> I now have the need to make my built in camera available in an App VM (I 
> choose untrusted, but may a dedicated one later on).
> 
> However, I am failing to pass through the
> USB controller to the App VM.

[snip]

I reread

https://www.qubes-os.org/doc/assigning-devices/

and tried enabling 'permissive' mode as described for R3.2 in the above
documentation. However, this per se doesn't work, as the target file
(/sys/bus/pci/drivers/pciback/permissive)
is not writeable, even for root and even when triggered through systemd.

However, I then compared the 'kernelopts' of 'sys-net' to those of 'untrusted',
and noted that 'iommu=soft swiotlb=8192' where missing in the latter. So
I added those, together with forcing 'pci_strictreset False'.

After rebooting the whole machine, untrusted has grabbed the usb hub and sees
the camera. The expected loss of a USB port due to the strange 'wiring' of the 
Lenovo X220 is acceptable to me; furthermore, I do plan to attach the pci 
device only when I know that I'll need the camera. 

[snip]

> 
> And xl dmesg shows:
> 
> XEN) [VT-D] It's disallowed to assign 0000:00:1a.0 with shared RMRR at 
> da8d5000 for Dom5.
> (XEN) XEN_DOMCTL_assign_device: assign 0000:00:1a.0 to dom5 failed (-1)
> 

For the record, xl dmesg is now telling me that 
[VT-D] It's risky to assign .. with shared RMRR at .. for Dom4

what ever that means.

I don't know which of the options / changes did the trick, but one or more
of the above seems to enable the camera in 'untrusted'.

Best regards,

Stefan

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