docs: Add VFIO usage example

Fixes: #117

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

docs/vfio.md

@@ -0,0 +1,82 @@
+# 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 guess.
+
+### 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:
+
+```
+$ sudo modprobe vfio_pci
+$ sudo modprobe vfio_iommu_type1 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
+```
+
+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 ~/clear-29160-kvm.img \
+    --console off \
+    --serial tty \
+    --cmdline "console=ttyS0 reboot=k panic=1 nomodules i8042.noaux i8042.nomux i8042.nopnp i8042.dumbkbd root=/dev/vda3" \
+    --cpus 4 \
+    --memory size=512M \
+    --device /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.
+
+