[module] update README to reflect VM->host changes

module/README.md

@@ -1,6 +1,9 @@
 This kernel module implements a basic interface to the IVSHMEM device for
 LookingGlass when using LookingGlass in VM->VM mode.
 
+Additionally, in VM->host mode, it can be used to generate a shared memory
+device on the host machine that supports dmabuf.
+
 ## Compiling (Manual)
 
 Make sure you have your kernel headers installed first, on Debian/Ubuntu use
@@ -12,12 +15,20 @@ Then simply run `make` and you're done.
 
 ### Loading
 
-This module requires the `uio` module to be loaded first, loading it is as
-simple as:
+For VM->VM mode, simply run:
 
-    modprobe uio
     insmod kvmfr.ko
 
+For VM->host mode with dmabuf, instead of creating a shared memory file, load
+this module with the parameter `static_size_mb`. For example, a 128 MB shared
+memory device can be created with:
+
+    insmod kvmfr.ko static_size_mb=128
+
+Multiple devices can be created by separating the sizes with commas. For
+example, `static_size_mb=128,64` would create two kvmfr devices: `kvmfr0`
+would be 128 MB and `kvmfr1` would be 64 MB.
+
 ## Compiling & Installing (DKMS)
 
 You can install this module into DKMS so that it persists across kernel
@@ -27,21 +38,73 @@ upgrades. Simply run:
 
 ### Loading 
 
-Simply modprobe the module:
+For VM->VM, simply modprobe the module:
 
     modprobe kvmfr
 
+For VM->host with dmabuf, modprobe with the parameter `static_size_mb`:
+
+    modprobe kvmfr static_size_mb=128
+
+Just like above, multiple devices can be created by separating the sizes
+with commas.
+
 ## Usage
 
 This will create the `/dev/kvmfr0` node that represents the KVMFR interface.
 To use the interface you need permission to access it by either creating a
-udev rule to ensure your user can read and write to it, or simply change it's
+udev rule to ensure your user can read and write to it, or simply change its
 ownership manually, ie:
 
     sudo chown user:user /dev/kvmfr0
 
+An example udev rule, which you can put in `/etc/udev/rules.d/99-kvmfr.rules`,
+is (replace `user` with your username):
+
+    SUBSYSTEM=="kvmfr", OWNER="user", GROUP="kvm", MODE="0660"
+
 Usage with looking glass is simple, you only need to specify the path to the
 device node, for example:
 
     ./looking-glass-client -f /dev/kvmfr0
 
+### VM->Host
+
+In VM->host mode, use this device in place of the shared memory file.
+
+For example, with `qemu`, you would use the following arguments:
+
+    -device ivshmem-plain,id=shmem0,memdev=looking-glass
+    -object memory-backend-file,id=looking-glass,mem-path=/dev/kvmfr0,size=128M,share=yes
+
+Note that the `size` argument must be the same size as what you passed
+to `static_size_mb` argument for the kernel module.
+
+#### `libvirt`
+
+With `libvirt`, you can use the following XML block:
+
+```xml
+<qemu:commandline>
+  <qemu:arg value='-device'/>
+  <qemu:arg value='ivshmem-plain,id=shmem0,memdev=looking-glass'/>
+  <qemu:arg value='-object'/>
+  <qemu:arg value='memory-backend-file,id=looking-glass,mem-path=/dev/kvmfr0,size=128M,share=yes'/>
+</qemu:commandline>
+```
+
+Remember to add `xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'` to
+the `<domain>`.
+
+On certain distros, running libvirt this way poses issues with apparmor
+and cgroups.
+
+For apparmor, in `/etc/apparmor.d/abstractions/libvirt-qemu`, append:
+
+    # Looking Glass
+    /dev/kvmfr0 rw,
+
+For cgroups, in `/etc/libvirt/qemu.conf`, uncomment the `cgroup_device_acl`
+block and add `/dev/kvmfr0` to the list. Then restart `libvirtd`:
+
+    sudo systemctl restart libvirtd.service