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Cloud Hypervisor VFIO HOWTO

VFIO (Virtual Function I/O) is a kernel framework that exposes direct device access to userspace. cloud-hypervisor, as many VMMs do, uses the VFIO framework to directly assign host physical devices to the guest workloads.

Direct Device Assignment with Cloud Hypervisor

To assign a device to a cloud-hypervisor guest, the device needs to be managed by the VFIO kernel drivers. However, by default, a host device will be bound to its native driver, which is not the VFIO one.

As a consequence, a device must be unbound from its native driver before passing it to cloud-hypervisor for assigning it to a guest.

Example

In this example we're going to assign a PCI memory card (SD, MMC, etc) reader from the host in a cloud hypervisor guest.

cloud-hypervisor only supports assigning PCI devices to its guests. lspci helps with identifying PCI devices on the host:

$ lspci
[...]
01:00.0 Unassigned class [ff00]: Realtek Semiconductor Co., Ltd. RTS525A PCI Express Card Reader (rev 01)
[...]

Here we see that our device is on bus 1, slot 0 and function 0 (01:00.0).

Now that we have identified the device, we must unbind it from its native driver (rtsx_pci) and bind it to the VFIO driver instead (vfio_pci).

First we add VFIO support to the host:

# modprobe -r vfio_pci
# modprobe -r vfio_iommu_type1
# modprobe vfio_iommu_type1 allow_unsafe_interrupts
# modprobe vfio_pci

In case the VFIO drivers are built-in, enable unsafe interrupts with:

# echo 1 > /sys/module/vfio_iommu_type1/parameters/allow_unsafe_interrupts

Then we unbind it from its native driver:

# echo 0000:01:00.0 > /sys/bus/pci/devices/0000\:01\:00.0/driver/unbind

And finally we bind it to the VFIO driver. To do that we first need to get the device's VID (Vendor ID) and PID (Product ID):

$ lspci -n -s 01:00.0
01:00.0 ff00: 10ec:525a (rev 01)

# echo 10ec 525a > /sys/bus/pci/drivers/vfio-pci/new_id

If you have more than one device with the same vendorID/deviceID, starting with the second device, the binding is performed as follows:

# echo 0000:02:00.0 > /sys/bus/pci/drivers/vfio-pci/bind

Now the device is managed by the VFIO framework.

The final step is to give that device to cloud-hypervisor to assign it to the guest. This is done by using the --device command line option. This option takes the device's sysfs path as an argument. In our example it is /sys/bus/pci/devices/0000:01:00.0/:

./target/debug/cloud-hypervisor \
    --kernel ~/vmlinux \
    --disk path=~/focal-server-cloudimg-amd64.raw \
    --console off \
    --serial tty \
    --cmdline "console=ttyS0 root=/dev/vda1 rw" \
    --cpus 4 \
    --memory size=512M \
    --device path=/sys/bus/pci/devices/0000:01:00.0/

The guest kernel will then detect the card reader on its PCI bus and provided that support for this device is enabled, it will probe and enable it for the guest to use.

In case you want to pass multiple devices, here is the correct syntax:

--device path=/sys/bus/pci/devices/0000:01:00.0/ path=/sys/bus/pci/devices/0000:02:00.0/

Multiple devices in the same IOMMU group

There are cases where multiple devices can be found under the same IOMMU group. This happens often with graphics card embedding an audio controller.

$ lspci
[...]
01:00.0 VGA compatible controller: NVIDIA Corporation GK208B [GeForce GT 710] (rev a1)
01:00.1 Audio device: NVIDIA Corporation GK208 HDMI/DP Audio Controller (rev a1)
[...]

This is usually exposed as follows through sysfs:

$ ls /sys/kernel/iommu_groups/22/devices/
0000:01:00.0  0000:01:00.1

This means these two devices are under the same IOMMU group 22. In such case, it is important to bind both devices to VFIO and pass them both through the VM, otherwise this could cause some functional and security issues.