qemu: scsi id conflict when both disk_interface and cdrom_interface are set to virtio-scsi

[hc-github-team-packer]

This issue was originally opened by @AmedeeBulle in hashicorp/packer#11211 and has been migrated to this repository. The original issue description is below.

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Overview of the Issue

When setting both disk_interface and cdrom_interface to virtio-scsi, they will use the same target and qemu will error out with:

qemu-kvm: -drive file=<iso file path>,if=none,index=0,id=cdrom0,media=cdrom: drive with bus=0, unit=0 (index=0) exists

As workaround, we can list the drives in qemuargs and use index=1 for the cdrom, but this is not very practical...

The use case for having both set to virtio-scsi is:

• I want/need to have my image based on scsi / sda (target / portability reason)
• aarch64 requires virtio-scsi for the cdrom (on x86_64 I use the default -- virtio -- so I don't have this issue)

Reproduction Steps

With the Buildfile hereunder:

PACKER_LOG=1 packer build virtio-scsi.json

Packer version

Packer v1.7.4

Simplified Packer Buildfile

{
  "builders":
    [
      {
      "type": "qemu",
      "iso_url": "http://yum.oracle.com/ISOS/OracleLinux/OL8/u4/aarch64/aarch64-boot-uek.iso",
      "iso_checksum": "e15006202045c2cb287d72e5c4e490d59698d7ef247ba5280bce09e886a4841c",
      "ssh_username": "root",
      "ssh_password": "root",
      "output_directory": "/data/packer/virtio-scsi",
      "disk_size": "1024",
      "format": "qcow2",
      "disk_interface": "virtio-scsi",
      "cdrom_interface": "virtio-scsi",
      "cpus": 2,
      "headless": "true",
      "memory": 1024,
      "vm_name": "system.qcow",
      "qemu_binary": "/usr/libexec/qemu-kvm",
      "qemuargs": [
        [ "-machine", "gic-version=3,accel=kvm" ]
      ]
      }
    ]
}

(any iso file will do)

Operating system and Environment details

Oracle Linux 8.4 on aarch64.
Kernel: 5.4.17-2102.204.4.2.el8uek.aarch64
qemu-kvm: 4.2.0

Log Fragments and crash.log files

==> qemu: Overriding default Qemu arguments with qemuargs template option...
2021/08/17 08:24:38 packer-builder-qemu plugin: Executing /usr/libexec/qemu-kvm: []string{"-smp", "cpus=2,sockets=2", "-drive", "if=none,file=/data/packer/virtio-scsi/system.qcow,id=drive0,cache=writeback,discard=ignore,format=qcow2", "-drive", "file=/home/opc/packer/packer_cache/ac5f4ad7b472d5c56ae4e15ce40b2c8c068280ee.iso,if=none,index=0,id=cdrom0,media=cdrom", "-boot", "once=d", "-machine", "gic-version=3,accel=kvm", "-vnc", "127.0.0.1:12", "-device", "virtio-scsi-pci,id=scsi0", "-device", "scsi-hd,bus=scsi0.0,drive=drive0", "-device", "virtio-net,netdev=user.0", "-device", "virtio-scsi-device", "-device", "scsi-cd,drive=cdrom0", "-netdev", "user,id=user.0,hostfwd=tcp::3291-:22", "-m", "1024M", "-name", "system.qcow"}
2021/08/17 08:24:38 packer-builder-qemu plugin: Started Qemu. Pid: 10303
2021/08/17 08:24:38 packer-builder-qemu plugin: Qemu stderr: qemu-kvm: -drive file=/home/opc/packer/packer_cache/ac5f4ad7b472d5c56ae4e15ce40b2c8c068280ee.iso,if=none,index=0,id=cdrom0,media=cdrom: drive with bus=0, unit=0 (index=0) exists