Knowledge: Using vsock

vsock provides point to point connectivity between host and guest. On IBM Z, it requires the following minimum levels: Linux kernel 4.8 (host and guest), QEMU 2.8, and libvirt v4.7.

Addressing

A socket address is defined as a combination of a 32-bit Context Identifier (CID) and a 32-bit port number:

• CID: Identifies the source or destination, which is either a virtual machine or the host. There are several special addresses:
• VMADDR_CID_ANY (-1U) means any address for binding
• VMADDR_CID_HYPERVISOR (0) is reserved for services built into the hypervisor
• VMADDR_CID_RESERVED (1) must not be used
• VMADDR_CID_HOST (2) is the well-known address of the host
• Port: Differentiates between multiple services running on a single machine.

 

Setup

Verify the kernel configs as follows:

• Host kernel:
    CONFIG_EVENTFD=y
    CONFIG_VSOCKETS=y
    CONFIG_VSOCKETS_DIAG=y
    CONFIG_VHOST_VSOCK=m
    CONFIG_VHOST=m
• Guest kernel:
    CONFIG_VIRTIO
    CONFIG_VSOCKETS=y
    CONFIG_VIRTIO_VSOCKETS=m
    CONFIG_VIRTIO_VSOCKETS_COMMON=m

Now load the module vhost_vsock in the host:

   user@host$ modprobe vhost_vsock

If SELinux is running: Unless you provide respective exception policies, make sure that it is set to 'permissive':

   user@host$ setenforce Permissive

Usage Example

Use streaming sockets of address family AF_VSOCK for vsock-based communication.
However, instead of writing your own BSD-sockets programs, you can simply use nc-vsock, a netcat clone. If not available in your linux distribution, download and build as follows in host and guest:

   user@host$ git clone https://github.com/stefanha/nc-vsock
   user@host$ cd nc-vsock
   user@host$ make

Next up, enable a guest domain by adding a vsock device as follows:

   <vsock model='virtio'>
     <cid auto='no' address='9'/>
     <address type='ccw' cssid='0xfe' ssid='0x0' devno='0x0003'/>
   </vsock>

Note that these parts can be skipped during guest definition.

Start a server on the host as follows:

   user@host$ nc-vsock -l 1234

Send a message from a guest to the host (represented by CID 2, see above):

   user@guest$ echo "This message is from guest $HOSTNAME" > guestmsg.txt
   user@guest$ nc-vsock 2 1234 < guestmsg.txt

Finally check the host's console to confirm that the message from the guest was received indeed:

   user@host$ nc-vsock -l 1234
   Connection from cid 9 port 1024...
   This message is from guest s38lp11