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.

Build virtiofsd

git clone https://gitlab.com/virtio-fs/virtiofsd
pushd virtiofsd
cargo build --release
sudo setcap cap_sys_admin+epi target/release/virtiofsd

Create shared directory

mkdir /tmp/shared_dir

Run virtiofsd

./virtiofsd \
    -d \
    --socket-path=/tmp/virtiofs \
    --shared-dir=/tmp/shared_dir \
    --cache=never

The cache=never option should be the default when using virtiofsd with the cloud-hypervisor VMM. This prevents from using the guest page cache, which reduces the memory footprint of the guest. When running multiple virtual machines on the same host, this will let the host deal with page cache, which will increase the density of virtual machines which can be launched.

The cache=always option will allow for the guest page cache to be used, which will increase the memory footprint of the guest. This option should be used only for specific use cases where a single VM is going to be running on a host.

Kernel support

Modern Linux kernels starting (at least v5.10) have support for virtio-fs. Use of older kernels, with additional patches, are not supported.

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 is the preferred option, but we can boot from an EFI cloud image if it contains a recent enough kernel.

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 must specify shared=on to share the memory pages so that an external process can access them.

Assuming you have focal-server-cloudimg-amd64.raw and vmlinux on your system, here is the cloud-hypervisor command you need to run:

./cloud-hypervisor \
    --cpus boot=1 \
    --memory size=1G,shared=on \
    --disk path=focal-server-cloudimg-amd64.raw \
    --kernel vmlinux \
    --cmdline "console=hvc0 root=/dev/vda1 rw" \
    --fs tag=myfs,socket=/tmp/virtiofs,num_queues=1,queue_size=512

By default, DAX is enabled with a cache window of 8GiB. You can specify a custom size (let’s say 4GiB for this example) for the cache by explicitly setting DAX and the cache size:

--fs tag=myfs,socket=/tmp/virtiofs,num_queues=1,queue_size=512,dax=on,cache_size=4G

In case you don’t want to use a shared window of cache to pass the shared files content, this means you will have to explicitly disable DAX with dax=off. Note that in this case, the cache_size parameter will be ignored.

--fs tag=myfs,socket=/tmp/virtiofs,num_queues=1,queue_size=512,dax=off

Mount the shared directory

The last step is to mount the shared directory inside the guest, using the virtiofs filesystem type.

mkdir mount_dir
mount -t virtiofs -o dax myfs mount_dir/

The tag needs to be consistent with what has been provided through the cloud-hypervisor command line, which happens to be myfs in this example.

The -o dax option must be removed in case the shared cache region is not enabled from the VMM.