1# Cloud Hypervisor VFIO HOWTO 2 3VFIO (Virtual Function I/O) is a kernel framework that exposes direct device 4access to userspace. `cloud-hypervisor`, as many VMMs do, uses the VFIO 5framework to directly assign host physical devices to the guest workloads. 6 7## Direct Device Assignment with Cloud Hypervisor 8 9To assign a device to a `cloud-hypervisor` guest, the device needs to be managed 10by the VFIO kernel drivers. However, by default, a host device will be bound to 11its native driver, which is not the VFIO one. 12 13As a consequence, a device must be unbound from its native driver before passing 14it to `cloud-hypervisor` for assigning it to a guest. 15 16### Example 17 18In this example we're going to assign a PCI memory card (SD, MMC, etc) reader 19from the host in a cloud hypervisor guest. 20 21`cloud-hypervisor` only supports assigning PCI devices to its guests. `lspci` 22helps with identifying PCI devices on the host: 23 24``` 25$ lspci 26[...] 2701:00.0 Unassigned class [ff00]: Realtek Semiconductor Co., Ltd. RTS525A PCI Express Card Reader (rev 01) 28[...] 29``` 30 31Here we see that our device is on bus 1, slot 0 and function 0 (`01:00.0`). 32 33Now that we have identified the device, we must unbind it from its native driver 34(`rtsx_pci`) and bind it to the VFIO driver instead (`vfio_pci`). 35 36First we add VFIO support to the host: 37 38``` 39# modprobe -r vfio_pci 40# modprobe -r vfio_iommu_type1 41# modprobe vfio_iommu_type1 allow_unsafe_interrupts 42# modprobe vfio_pci 43``` 44 45In case the VFIO drivers are built-in, enable unsafe interrupts with: 46 47``` 48# echo 1 > /sys/module/vfio_iommu_type1/parameters/allow_unsafe_interrupts 49``` 50 51Then we unbind it from its native driver: 52 53``` 54# echo 0000:01:00.0 > /sys/bus/pci/devices/0000\:01\:00.0/driver/unbind 55``` 56 57And finally we bind it to the VFIO driver. To do that we first need to get the 58device's VID (Vendor ID) and PID (Product ID): 59 60``` 61$ lspci -n -s 01:00.0 6201:00.0 ff00: 10ec:525a (rev 01) 63 64# echo 10ec 525a > /sys/bus/pci/drivers/vfio-pci/new_id 65``` 66 67If you have more than one device with the same `vendorID`/`deviceID`, starting 68with the second device, the binding is performed as follows: 69 70``` 71# echo 0000:02:00.0 > /sys/bus/pci/drivers/vfio-pci/bind 72``` 73 74Now the device is managed by the VFIO framework. 75 76The final step is to give that device to `cloud-hypervisor` to assign it to the 77guest. This is done by using the `--device` command line option. This option 78takes the device's sysfs path as an argument. In our example it is 79`/sys/bus/pci/devices/0000:01:00.0/`: 80 81``` 82./target/debug/cloud-hypervisor \ 83 --kernel ~/vmlinux \ 84 --disk path=~/focal-server-cloudimg-amd64.raw \ 85 --console off \ 86 --serial tty \ 87 --cmdline "console=ttyS0 root=/dev/vda1 rw" \ 88 --cpus 4 \ 89 --memory size=512M \ 90 --device path=/sys/bus/pci/devices/0000:01:00.0/ 91``` 92 93The guest kernel will then detect the card reader on its PCI bus and provided 94that support for this device is enabled, it will probe and enable it for the 95guest to use. 96 97In case you want to pass multiple devices, here is the correct syntax: 98 99``` 100--device path=/sys/bus/pci/devices/0000:01:00.0/ path=/sys/bus/pci/devices/0000:02:00.0/ 101``` 102 103### Multiple devices in the same IOMMU group 104 105There are cases where multiple devices can be found under the same IOMMU group. 106This happens often with graphics card embedding an audio controller. 107 108``` 109$ lspci 110[...] 11101:00.0 VGA compatible controller: NVIDIA Corporation GK208B [GeForce GT 710] (rev a1) 11201:00.1 Audio device: NVIDIA Corporation GK208 HDMI/DP Audio Controller (rev a1) 113[...] 114``` 115 116This is usually exposed as follows through `sysfs`: 117 118``` 119$ ls /sys/kernel/iommu_groups/22/devices/ 1200000:01:00.0 0000:01:00.1 121``` 122 123This means these two devices are under the same IOMMU group 22. In such case, 124it is important to bind both devices to VFIO and pass them both through the 125VM, otherwise this could cause some functional and security issues.