How to start a qemu virtual machine and from the host, execute some commands to finally shut it down

Question

I'm deploying virtual machines with qemu. Installed OS is Debian with linux kernel debian 6.8.11-amd64. The way I deploy a vm is directly with qemu-system command (i.e no vm manager).

In a command fashion, I would like -from the host- to:

1. Start the VM
2. Execute a serie of command within the guest (git clone something, configure something, make something, execute something, return success!)
3. Shutdown the VM

I found that one can exchange with the VM through QMP using specific command,
(https://qemu-project.gitlab.io/qemu/interop/qemu-qmp-ref.html#id2)

But I cant find how to do my step 2. It looks like only a specific set of commands are authorized with QMP..

Thanks in advance,

Pierre

Answer 1

You will need to build a custom filesystem image that includes scripts and/or some other tooling that will run your commands and power off the virtual machine. There are many ways to approach this problem. Here's one possible solution:

1. Start with an operating system image. For this example, I'm using the debian 12 cloud image.

2. We'll create a copy-on-write clone of the image to hold our changes. This makes it easy to start over. Here we're creating a 5 GB image; you could adjust the image size as necessary for your workload:

   qemu-img create -f qcow2 -F qcow2 \
     -b debian-12-genericcloud-amd64.qcow2 root.qcow2 5g
   

3. Now we need to embed some additional information in the image. For this example, we need a netplan file to enable DHCP, and an rc.local script that will perform our clone/build/install/etc actions.

   I have a netplan file called network.yaml that looks like this:

   network:
     version: 2
     renderer: networkd
     ethernets:
       ens3:
         dhcp4: true
   

   And an rc.local file that looks like this:

   #!/bin/sh
   
   set -e
   
   while ! ping -c1 dns.google; do
     echo "waiting for network"
     sleep 1
   done
   
   apt-get -y update
   DEBIAN_FRONTEND=noninteractive apt-get -y install gcc make autoconf automake libtool git autopoint gperf texinfo help2man
   
   mkdir -p /srv/build
   cd /srv/build
   git clone https://git.savannah.gnu.org/git/hello.git
   cd hello
   ./bootstrap
   ./configure --prefix=/usr/local
   make
   make install
   /usr/local/bin/hello
   
   poweroff
   

   We can use the virt-customize command to embed these in the image:

   virt-customize -a root.qcow2 \
     --copy-in network.yaml:/etc/netplan/ \
     --copy-in rc.local:/etc/
   

4. Now we can use the qemu-system-x86_64 command to boot our image:

   qemu-system-x86_64 -m 1g -enable-kvm \
     -nographic \
     -serial mon:stdio \
     -drive file=root.qcow2,driver=qcow2 \
     -nic user,model=virtio-net-pci
   

   This will run the commands in our rc.local file and power off the vm when complete.

If you need to explicitly report results back to the host, you again have lots of options. You could add a second serial port to your virtual machine (-serial file:status.txt) and write status data there (echo OK > /dev/ttyS1), or you could capture the console output to a file (replace -serial mon:stdio with -serial file:console.txt) and then grep the output when the vm complete, etc.

If you are concerned about the script not completing due to an error, you could wrap your qemu-system-x86_64 command with the timeout command. For example, to ensure the qemu vm exits in under 2 minutes:

timeout 120 qemu-system-x86_64 -m 1g -enable-kvm ...