mirror of
https://github.com/cloud-hypervisor/cloud-hypervisor.git
synced 2026-08-05 02:19:16 +00:00
042d1abd67
Include documentation for the generic vhost-user device. Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
77 lines
2.8 KiB
Markdown
77 lines
2.8 KiB
Markdown
# How to use generic vhost-user devices
|
|
|
|
## What is a generic vhost-user device?
|
|
|
|
Cloud Hypervisor deliberately does not have support for all types of virtio devices.
|
|
For instance, it does not natively support sound or media.
|
|
|
|
However, the vhost-user protocol does not require the frontend to have separate
|
|
code for each type of vhost-user device. This allows writing a *generic* frontend
|
|
that supports almost all of them.
|
|
|
|
Any vhost-user device that only uses supported protocol messages is
|
|
expected to work. It can (and often will) be of a type that Cloud
|
|
Hypervisor does not know about. It can even be of a type that is
|
|
not standardized.
|
|
|
|
Virtio-GPU is known to *not* work. The version implemented in QEMU
|
|
requires `VHOST_USER_GPU_SET_SOCKET`, which is standard but will
|
|
never be implemented by Cloud Hypervisor. Other versions require
|
|
messages that have not been standardized. In the future, these
|
|
versions might be supported.
|
|
|
|
## Examples
|
|
|
|
virtiofsd meets these requirements if the `--tag` argument is passed.
|
|
Therefore, generic vhost-user can be used as an alternative to the built-in
|
|
virtio-fs support. See [fs.md](fs.md) for how to build the virtiofs daemon.
|
|
|
|
To use generic vhost-user with virtiofsd, use a command line argument
|
|
similar to this:
|
|
|
|
```bash
|
|
/path/to/virtiofsd \
|
|
--tag=myfs \
|
|
--log-level=debug \
|
|
"--socket-path=$path_to_virtiofsd_socket" \
|
|
"--shared-dir=$path_to_shared_directory" \
|
|
"${other_virtiofsd_options[@]}" &
|
|
|
|
/path/to/cloud-hypervisor \
|
|
--cpus boot=1 \
|
|
--memory size=1G,shared=on \
|
|
--disk path=your-linux-image.iso \
|
|
--kernel vmlinux \
|
|
--cmdline "console=hvc0 root=/dev/vda1 rw" \
|
|
--generic-vhost-user "socket=\"${path_to_virtiofsd_socket//\"/\"\"}\",virtio_id=26,queue_sizes=[512,512]" \
|
|
"${other_cloud_hypervisor_options[@]}"
|
|
```
|
|
|
|
26 is the ID for a virtio-fs device. The IDs for other devices are defined
|
|
by the VIRTIO specification. The odd-looking variable expansion escapes
|
|
any double quotes in the socket path. It is also possible to provide
|
|
the name that is defined by the virtio specification, so `virtio_id=fs`
|
|
will also work.
|
|
|
|
Inside the guest, you can mount the virtio-fs device with
|
|
|
|
```bash
|
|
mkdir mount_dir
|
|
mount -t virtiofs -- myfs mount_dir/
|
|
```
|
|
|
|
## Limitations
|
|
|
|
Cloud Hypervisor does not save, restore, or migrate the PCI configuration
|
|
space of a generic vhost-user device. The backend can do it itself, but if
|
|
it does not these features will not work.
|
|
|
|
Cloud Hypervisor cannot validate the number or size of the queues. Some
|
|
guest drivers do not validate these and will crash if they are wrong.
|
|
Notably, at least some versions of Linux will crash if one creates a
|
|
virtio-fs device (id 26) with only one queue.
|
|
|
|
If any access to configuration space fails, Cloud Hypervisor will panic
|
|
instead of injecting an exception into the guest. It is unclear what
|
|
correct behavior is in this case.
|