Add support for additional QEMU command line parameters

[Sam13]

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Description

Currently it's only possible to customize the QEMU command line via the qemuargs option (documented under https://developer.hashicorp.com/packer/integrations/hashicorp/qemu/latest/components/builder/qemu#optional).

By using this some Packer generated command line switches are omitted (e.g. disks).

This is especially annoying when an additional drive needs to be added together with the CD configuration (https://developer.hashicorp.com/packer/integrations/hashicorp/qemu/latest/components/builder/qemu#cd-configuration) for cloud init. Since CD ROM ISO generated on the fly it's impossible to obtain this path to the ISO and mount it via qemuargs.

Proposal of this feature request is to add an additional configuration parameter which is appended to the Packer generated QEMU command line as is.

Use Case(s)

• Adding an additional drive
• Customize other QEMU behavior

Potential configuration

qemu_additional_args = [
  ["-drive", "file=/tmp/my-additional-drive.qcow,if=virtio"]
]

[computerquip-work]

I'm currently in this boat as well. If you use cd_files and want to do something like mount virtio's driver ISO, then the resulting ISO from cd_files that's supposed to be mounted is just... not. packer places the iso in a random location in /tmp so I appear to be SOL.

[tatsushid]

I struggled for adding an extra ISO drive for installing VirtIO drivers at creating Windows image for example like commented above but I found a workaround not to cause conflict between a Packer generated option and a qemuargs custom option

Packer QEMU plugin only checks a single hyphen prefixed option like -drive, -device, -chardev etc. in its code (basically in builder/qemu/step_run.go) while QEMU runner like qemu-system-x86_64 can also accept a double hyphen prefixed option like --drive, --device, --chardev etc. as same meaning as a single hyphen one so when I specify a double hyphen prefixed option in qemuargs, it is active and it doesn't overwrite a Packer generated option

This QEMU runner behavior isn't described in a document as far as I check so might not be supported in the future but it works on Ubuntu 24.04 and its official QEMU package (version 8.2.2) at least. I haven't checked the other version, distribution etc.

I also think it's nice to have something an option to customize QEMU runner flexibly for this plugin but some kind of customization problems by qemuargs can be avoided by this workaround