vsock is pretty widely used, and if you're using virtio-vsock it should be reasonably fast. Anyway if you want to do some quick benchmarks and have an existing Linux VM on a libvirt host:

(1) 'virsh edit' the guest and check it has '<vsock/>' in the <devices> section of the XML.

(2) On the host:

  $ nbdkit memory 1G --vsock -f

(3) Inside the guest:

  $ nbdinfo 'nbd+vsock://2'

(You should see the size being 1G)

And then you can try using commands like nbdcopy to copy data into and out of the host RAM disk over vsock. eg:

  $ time nbdcopy /dev/urandom 'nbd+vsock://2' -p
  $ time nbdcopy 'nbd+vsock://2' null: -p

On my machine that's copying at a fairly consistent 20 Gbps, but it's going to depend on your hardware.

To compare it to regular TCP:

  host $ nbdkit memory 1G -f -p 10809
  vm $ time nbdcopy /dev/urandom 'nbd://host' -p
  vm $ time nbdcopy 'nbd://host' null: -p

TCP is about 2.5x faster for me.

I had to kill the firewall on my host to do the TCP test (as trying to reconfigure nft/firewalld was beyond me), which actually points to one advantage of vsock, it bypasses the firewall. It's therefore convenient for things like guest agents where you want them to "just work" without reconfiguration hassle.

It's probably faster than, say, an emulated UART port.

But likely no faster than a TCP socket across a virtio-net device.

> Given how slow protobufs and grpc is, I wonder if the socket transport would ever be the bottleneck to throughput here.

I think this is supposed to be option for when you want to pass stuff to host quickly without writing another device driver or using other interface rather than replacement for any rpc between VMs. "Being fast" is just a bonus.

For example at our job we use serial port for the communication with VM agent (it's just passing some host info about where VM is running, so our automation system can pick it up), this would be ideal replacement for that.

And as it is "just a socket", stuff like this is pretty easy to setup https://libvirt.org/ssh-proxy.html

> This is a weird article. It’s titled “how to use vsock” but 95% is how to set up Bazel, gRPC, and building a C++ project.

This is just what happens when you use bazel to quickly set up your project.