docs: Add virtio-fs documentation

Add some documentation specific to virtio-fs and how to perform
filesystem sharing between host and guest with cloud-hypervisor.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

docs/fs.md

@@ -0,0 +1,70 @@
+# How to use virtio-fs
+
+In the context of virtualization, it is always convenient to be able to share a directory from the host with the guest.
+
+__virtio-fs__, also known as __vhost-user-fs__ is a virtual device defined by the VIRTIO specification which allows any VMM to perform filesystem sharing.
+
+## Pre-requisites
+
+### The daemon
+
+This virtual device relies on the _vhost-user_ protocol, which assumes the backend (device emulation) is handled by a dedicated process running on the host. This daemon is called __virtiofsd__ and needs to be present on the host.
+
+_Install virtiofsd_
+```bash
+VIRTIOFSD_URL="$(curl --silent https://api.github.com/repos/intel/nemu/releases/latest | grep "browser_download_url" | grep "virtiofsd-x86_64" | grep -o 'https://.*[^ "]')"
+wget --quiet $VIRTIOFSD_URL -O "virtiofsd"
+chmod +x "virtiofsd"
+sudo setcap cap_sys_admin+epi "virtiofsd"
+```
+_Create shared directory_
+```bash
+mkdir /tmp/shared_dir
+```
+_Run virtiofsd_
+```bash
+./virtiofsd \
+    -d \
+    -o vhost_user_socket=/tmp/virtiofs \
+    -o source=/tmp/shared_dir \
+    -o cache=none
+```
+The `cache=none` option here is an important one as it tells the daemon not to try any memory mapping of the files, but instead to use the _virtqueues_ to convey the files content. The support for the memory mapping of the files will be added later.
+
+### The kernel
+
+In order to leverage __virtio-fs__ support from within the guest, and because the code has not been merged in upstream Linux kernel yet, it is required to build a custom kernel embedding the patches.
+
+The following branch `virtio-pmem_and_virtio-fs` on the repository https://github.com/sboeuf/linux.git includes all the needed patches to support __virtio-fs__.
+
+Make sure to build a kernel out of this branch that can be then used to boot the VM.
+
+## How to share directories with cloud-hypervisor
+
+### Start the VM
+Once the daemon is running, the option `--fs` from __cloud-hypervisor__ needs to be used.
+
+Direct kernel boot option is preferred since we need to provide the custom kernel including the __virtio-fs__ patches. We could boot from `hypervisor-fw` if we had previously edited the image to replace the kernel binary.
+
+Because _vhost-user_ expects a dedicated process (__virtiofsd__ in this case) to be able to access the guest RAM to communicate through the _virtqueues_ with the driver running in the guest, `--memory` option needs to be slightly modified. It needs to specify a backing file for the memory so that an external process can access it.
+
+Assuming you have `clear-kvm.img` and `custom-vmlinux.bin` on your system, here is the __cloud-hypervisor__ command you need to run:
+```bash
+./cloud-hypervisor \
+    --cpus 4 \
+    --memory "size=512,file=/dev/shm" \
+    --disk clear-kvm.img \
+    --kernel custom-vmlinux.bin \
+    --cmdline "console=ttyS0 reboot=k panic=1 nomodules root=/dev/vda3" \ 
+    --fs tag=virtiofs,sock=/tmp/virtiofs,num_queues=1,queue_size=512
+```
+
+### Mount the shared directory
+The last step is to mount the shared directory inside the guest, using the `virtio_fs` filesystem type.
+```bash
+mkdir mount_dir
+mount \
+    -t virtio_fs /dev/null mount_dir/ \
+    -o tag=virtiofs,rootmode=040000,user_id=0,group_id=0
+```
+The `tag` needs to be consistent with what has been provided through the __cloud-hypervisor__ command line.