migration guide from socket_vmnet

[tamird]

I implemented support for socket_vmnet in Fuchsia's QEMU wrapper script:

https://cs.opensource.google/fuchsia/fuchsia/+/main:zircon/scripts/run-zircon;l=396-405;drc=335be9e7610ad8621f9b3a6081875a8fab4a53ba

it would be neat to replace that usage with vmnet-helper, which doesn't suffer some of the architectural problems of socket_vmnet, but it looks a bit more complicated to hold. Can you write an example of how to invoke QEMU under socket_vmnet? The simpler the better, as this script is unfortunately written in bash, not python.

[nirs]

The only way to integrate with qemu is using --fd:

vmnet-helper/vmnet/vm.py

Line 159 in 56935f2

elif self.socket is not None:

If may be possible to use 2 sockets if we change the helper to use 2 sockets instread of one, but I don't think it make sense. It is better to change QEMU to support single socket in addition to the way it works now.

I don't think you can create a socketpair in bash so you need to use something else for the wrapper. You can write a simple wrapper in Go, see the minikube vmnet integration for example:
https://github.com/kubernetes/minikube/blob/2aaecb0a747ae05d21789904aa138edbd3556580/pkg/drivers/vfkit/vfkit.go#L223

[tamird]

I think we really want to avoid having a build step for whatever we do there, in Fuchsia. It's not a deal breaker, but it is a lot more work relative to modifying the existing bash script (even if means interposing a python script for the socketpair creation).

[nirs]

So why not use python for the script, or at least for the part starting qemu?

[tamird]

Yeah, we can. It would be useful to see a minimal example of it. I personally find the code in https://github.com/nirs/vmnet-helper/blob/main/example -> https://github.com/nirs/vmnet-helper/tree/main/vmnet hard to follow.

[nirs]

Yes, this code try to support 3 drivers with both --fd and --socket on 3 distros. This is useful for testing but not very clear example.

[nirs]

I can post a minimal python example later.

[nirs]

Looking in the current code:

QEMU_WRAPPER=("$(brew --prefix socket_vmnet)/bin/socket_vmnet_client" "$SOCKET_VMNET_SOCKET")
ARGS+=("-netdev" "socket,id=net0,fd=3")

Maybe we are missing something like socket_vmnet_client that will be part of vment-helper installation? This can be a tiny program creating the socketpair, starting vment-helper, and running qemu with fd=3.

[tamird]

Yep, such a program would be very useful.

[nirs]

vment-client

Command line

vment-client [--interface-id ID] [--operation-mode MODE] [--shared-inteface IFACE]
             -- command ...

Options:
  --interface-id       if set, keep the same mac address for same id
  --operation-mode     vmnet operation mode [shared, bridged, host] (default shared) 
  --shared-interface   required if using bridged mode

Example qemu usage

Using qemu command line from integration test:
https://github.com/nirs/vmnet-helper/actions/runs/13608770252/job/38043387504

vment-client -- \
    qemu-system-x86_64 \
    -name \
    test \
    -m \
    1024 \
    -cpu \
    host \
    -machine \
    q35,accel=hvf \
    -smp \
    1,sockets=1,cores=1,threads=1 \
    -drive \
    if=pflash,format=raw,readonly=on,file=/usr/local/share/qemu/edk2-x86_64-code.fd \
    -drive \
    file=/Users/runner/.vmnet-helper/vms/test/disk.img,if=virtio,format=raw,discard=on \
    -drive \
    file=/Users/runner/.vmnet-helper/vms/test/cidata.iso,id=cdrom0,if=none,format=raw,readonly=on \
    -device \
    virtio-scsi-pci,id=scsi0 \
    -device \
    scsi-cd,bus=scsi0.0,drive=cdrom0 \
    -netdev \
    dgram,id=net1,local.type=fd,local.str=4 \
    -device \
    virtio-net-pci,netdev=net1,mac=1a:ad:75:f7:ca:a2 \
    -monitor \
    none \
    -serial \
    file:/Users/runner/.vmnet-helper/vms/test/serial.log \
    -nographic \
    -nodefaults \
    -kernel \
    /Users/runner/.vmnet-helper/vms/test/kernel \
    -append \
    root=LABEL=cloudimg-rootfs ro console=tty1 console=ttyS0 apic=debug no_timer_check \
    -initrd \
    /Users/runner/.vmnet-helper/vms/test/initrd

This command will obtain new mac address for every run, even if you use the same vm name.

vmnet-client flow

1. Parse options
2. Create socket pair (fd=3, fd=4)
3. fork
   • child: execve vmnet-helper with --fd=3 and helper options
   • parent: execve command

@tamird what do think?

[nirs]

@tamird Added vmnet-client in #63. Do you want to review and test it?

[tamird]

Sure. Will get to it on Monday.