Assigning Devices to VMs

In order to assign a whole PCI(e) device to a VM, one should use the qvm-pci tool. First, list the available PCI devices:

lspci

This will show you the BDF address of each PCI device. It will look something like 00:1a.0. Once you’ve found the BDF address of the device you want to assign, then attach it like so:

qvm-pci -a <vmname> <bdf>

For example, if 00:1a.0 is the BDF of the device I want to assign to the “personal” domain, I would do this:

qvm-pci -a personal 00:1a.0

Note that one can only assign full PCI or PCI Express devices. This means one cannot assign single USB devices – only the whole USB controller with whatever USB devices are connected to it. This limit is imposed by the PC and VT-d architectures. More information on using and managing USB devices with qubes is available on the USB page.

While a device can only be attached to one VM at a time, it is possible to assign the same device to more than one VM at a time. This means that you can use the device in one VM, shut that VM down, start up a different VM (to which the same device is also assigned), then use the device in that VM. This can be useful if, for example, you have only one USB controller, but you have multiple security domains which all require the use of different USB devices.

Using Qubes Manager

The above steps can also be done in Qubes Manager. Simply go into the VM settings of your desired VM, then go to the “Devices” tab. This will show you a list of available devices, which you can select to be assigned to that VM.

Finding the right USB controller

If you want assign a certain USB device to a VM (by attaching the whole USB controller), you need to figure out which PCI device is the right controller. First, check to which USB bus the device is connected:

lsusb

For example, I want assign a broadband modem to the netvm. In the out put of lsusb it can be listed as something like this. (In this case, the device isn’t fully identified):

Bus 003 Device 003: ID 413c:818d Dell Computer Corp.

The device is connected to USB bus #3. Then check which other devices are connected to the same bus, since all of them will be assigned to the same VM. Now is the time to find right USB controller:

readlink /sys/bus/usb/devices/usb3

This should output something like:

../../../devices/pci-0/pci0000:00/0000:00:1a.0/usb3

Now you see the BDF address in the path (right before final usb3). Strip the leading 0000: and pass the rest to the qvm-pci tool:

qvm-pci -a netvm 00:1a.0

Possible issues

DMA buffer size

VMs with assigned PCI devices in Qubes have allocated a small buffer for DMA operations (called swiotlb). By default it is 2MB, but some devices need a larger buffer. To change this allocation, edit VM’s kernel parameters (this is expressed in 512B chunks):

# qvm-prefs netvm |grep kernelopts
kernelopts       : iommu=soft swiotlb=2048 (default)
# qvm-prefs -s netvm kernelopts "iommu=soft swiotlb=4096"

This is known to be needed for the Realtek RTL8111DL Gigabit Ethernet Controller.

PCI passthrough issues

Sometimes PCI arbitrator is too strict. There is a way to enable permissive mode for it. Create /etc/systemd/system/qubes-pre-netvm.service:

[Unit]
Description=Netvm fixup
Before=qubes-netvm.service

[Service]
ExecStart=/bin/sh -c 'echo 0000:04:00.0 > /sys/bus/pci/drivers/pciback/permissive'
Type=oneshot
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Then enable it with systemctl enable qubes-pre-netvm.service

See also: this thread and the Xen wiki’s PCI passthrough page.

NOTE: By setting the permissive flag for the PCI device, you’re potentially weakening the device isolation, especially if your system is not equipped with VT-d Interrupt Remapping unit. See Software Attacks on Intel VT-d (page 7) for more details.

Bringing PCI device back to dom0

By default, when a device is detached from a VM (or when a VM with an attached PCI device is shut down), the device is not automatically attached back to dom0. This is an intended feature. A device which was previously assigned to a VM less trusted than dom0 (which, in Qubes, is all of them) could attack dom0 if it were automatically reassigned there.

In order to re-enable the device in dom0, either:

  • Reboot the physical machine.

or

  • Go to the sysfs (/sys/bus/pci), find the right device, detach it from the pciback driver, and attach it back to the original driver. Replace <BDF> with your device, for example 00:1c.2:

    echo 0000:<BDF> > /sys/bus/pci/drivers/pciback/unbind
    MODALIAS=`cat /sys/bus/pci/devices/0000:<BDF>/modalias`
    MOD=`modprobe -R $MODALIAS | head -n 1`
    echo <BDF> > /sys/bus/pci/drivers/$MOD/bind