Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com> Co-authored-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
3.4 KiB
VSOCK support
VSOCK provides a way for guest and host to communicate through a socket. VSOCK sockets support both stream and datagram types.
The virtio-vsock
is based on the Firecracker implementation, where additional details can be found.
What is a CID?
CID is a 32-bit context identifier describing the source or destination. In combination with the port, the complete addressing can be achieved to describe multiple listeners running on the same machine.
The table below depicts the well known CID values:
CID | Description |
---|---|
-1 | Random CID |
0 | Hypervisor |
1 | Loopback |
2 | Host |
Prerequisites
Kernel Requirements
Host kernel: CONFIG_VHOST_VSOCK
Guest kernel: CONFIG_VIRTIO_VSOCKETS
Nested VM support
Linux v5.5 or newer is required for the L1 VM.
Loopback support
Linux v5.6 or newer is required.
Establishing VSOCK Connection
VSOCK device becomes available with --vsock
option passed by the VM start. Cloud Hypervisor can be invoked for instance as below:
cloud-hypervisor \
--cpus boot=1 \
--memory size=4G \
--firmware CLOUDHV.fd \
--disk path=jammy-server-cloudimg.raw \
--vsock cid=3,socket=/tmp/ch.vsock
The examples use socat >=1.7.4
to illustrate the VSOCK functionality. However, there are other tools supporting VSOCK, like ncat.
Connecting from Host to Guest
The host starts to listen on the defined port:
$ socat - VSOCK-LISTEN:1234
Once the host is listening, the guest can send data:
`echo -e "CONNECT 1234\nHello from host!" | socat - UNIX-CONNECT:/tmp/ch.vsock
Note the string CONNECT <port>
prepended to the actual data. It is possible for the guest to start listening on different ports, thus the specific command is needed to instruct VSOCK to which listener the host wants to connect. It needs to be sent once per connection. Once the connection established, data transfers can take place directly.
Connecting from Guest to Host
This first requires a listening UNIX socket on the host side. The UNIX socket path has to be constructed by using the socket path used at the VM launch time with appended _
and the port number to be used on the guest side. As in the example above, if we'd intended to connect from the guest to the port 1234
, the Unix socket path on the host side would be /tmp/ch.vsock_1234
.
Also note that the CID used on the guest side is the well known CID value 2
.
Listening on the host side:
$ socat - UNIX-LISTEN:/tmp/ch.vsock_1234
From the guest:
$ echo -e "Hello from guest!" | socat - VSOCK-CONNECT:2:1234
Links
- virtio-vsock in QEMU, Firecracker and Linux: Status, Performance and Challenges
- Leveraging virtio-vsock in the cloud and containers
- VSOCK man page
- https://stefano-garzarella.github.io/posts/2020-02-20-vsock-nested-vms-loopback/
- https://github.com/firecracker-microvm/firecracker/blob/main/docs/vsock.md