Host Kernel: rawhide 4.13.0-0.rc6.git4.2.fc28.x86_64 (on Fedora 24)

QEMU is mainline built from sources: QEMU emulator version 2.10.50 (v2.10.0-105-g223cd0e)

Guest: clear-17460-kvm.img (which has vsock support)

/usr/local/bin/qemu-system-x86_64 \
    -device vhost-vsock-pci,id=vhost-vsock-pci0,guest-cid=3 \
    -enable-kvm \
    -bios OVMF.fd \
    -smp sockets=1,cpus=4,cores=2 -cpu host \
    -m 1024 \
    -vga none -nographic \
    -drive file="$IMAGE",if=virtio,aio=threads,format=raw \
    -netdev user,id=mynet0,hostfwd=tcp::${VMN}0022-:22,hostfwd=tcp::${VMN}2375-:2375 \
    -device virtio-net-pci,netdev=mynet0 \
    -debugcon file:debug.log -global isa-debugcon.iobase=0x402 $@

Testing with: https://github.com/mdlayher/vsock/tree/master/cmd/vscp

On the host you run: vscp -v -r -p 1024 cpuinfo.txt

On the guest: vscp -v -s -c 2 -p 1024 /proc/cpuinfo

And you will see the file contents show up

Note: for -c the parameter should be < the