cpu_type no longer accepts flags (e.g. "host,flags=...") since 1.2.3

[TuncTaylan]

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Overview of the Issue

Before version 1.2.3, Packer allowed setting custom CPU flags alongside cpu_type.
For example, this configuration used to work:
cpu_type = "host,flags=+md-clear;+pcid;+spec-ctrl;+ssbd;+aes"
After upgrading to 1.2.3, this is no longer possible — Packer fails with the flags.
There’s currently no alternative way to pass CPU flags to the VM, which breaks existing builds that rely on specific feature flags.

Reproduction Steps

Use Packer < 1.2.3 with cpu_type = "host,flags=+aes" → works fine.
Upgrade to Packer ≥ 1.2.3 with the same configuration → error.

Plugin and Packer version

Packer v1.14.2

Simplified Packer Buildfile

source "proxmox-iso" "ubuntu-cloud-image" {
  # Required
  proxmox_url = var.proxmox_api_url
  username    = var.proxmox_api_token_id
  token       = var.proxmox_api_token_secret
  node        = var.node

  task_timeout = var.task_timeout

  boot_iso {
    iso_url          = "https://releases.ubuntu.com/${var.ubuntu_release}/ubuntu-${var.ubuntu_version}-live-server-amd64.iso"
    iso_storage_pool = var.iso_storage_pool
    iso_checksum     = var.iso_checksum
    iso_download_pve = var.iso_download_pve
    unmount          = var.unmount
  }

  # Optional
  insecure_skip_tls_verify = var.skip_tls_verify
  vm_name                  = local.name
  cpu_type                 = "host,flags=+md-clear;+pcid;+spec-ctrl;+ssbd;+aes"
  os                       = var.os
  ...
}

Operating system and Environment details

OS, Architecture, and any other information you can provide about the
environment.

Log Fragments and crash.log files

==> ubuntu-cloud-image-2025-10-17.proxmox-iso.ubuntu-cloud-image: Error creating VM: cpuType can only be one of the following values: 486, Broadwell, Broadwell-IBRS, Broadwell-noTSX, Broadwell-noTSX-IBRS, Cascadelake-Server, Cascadelake-Server-V2, Cascadelake-Server-V4, Cascadelake-Server-V5, Cascadelake-Server-noTSX, Conroe, Cooperlake, Cooperlake-V2, EPYC, EPYC-IBPB, EPYC-Milan, EPYC-Rome, EPYC-Rome-v2, EPYC-v3, Haswell, Haswell-IBRS, Haswell-noTSX, Haswell-noTSX-IBRS, Icelake-Client, Icelake-Client-noTSX, Icelake-Server, Icelake-Server-noTSX, Icelake-Server-v3, Icelake-Server-v4, Icelake-Server-v5, Icelake-Server-v6, IvyBridge, IvyBridge-IBRS, KnightsMill, Nahalem, Nahalem-IRBS, Opteron_G1, Opteron_G2, Opteron_G3, Opteron_G4, Opteron_G5, Penrym, SandyBridge, SandyBridge-IBRS, SapphireRapids, Skylake-Client, Skylake-Client-IBRS, Skylake-Client-noTSX-IBRS, Skylake-Client-v4, Skylake-Server, Skylake-Server-IBRS, Skylake-Server-noTSX-IBRS, Skylake-Server-v4, Skylake-Server-v5, Westmere, Westmere-IBRS, athlon, core2duo, coreduo, host, kvm32, kvm64, max, pentium, pentium2, pentium3, phenom, qemu32, qemu64, x86-64-v2, x86-64-v2-AES, x86-64-v3, x86-64-v4
Build 'ubuntu-cloud-image-2025-10-17.proxmox-iso.ubuntu-cloud-image' errored after 14 seconds 92 milliseconds: Error creating VM: cpuType can only be one of the following values: 486, Broadwell, Broadwell-IBRS, Broadwell-noTSX, Broadwell-noTSX-IBRS, Cascadelake-Server, Cascadelake-Server-V2, Cascadelake-Server-V4, Cascadelake-Server-V5, Cascadelake-Server-noTSX, Conroe, Cooperlake, Cooperlake-V2, EPYC, EPYC-IBPB, EPYC-Milan, EPYC-Rome, EPYC-Rome-v2, EPYC-v3, Haswell, Haswell-IBRS, Haswell-noTSX, Haswell-noTSX-IBRS, Icelake-Client, Icelake-Client-noTSX, Icelake-Server, Icelake-Server-noTSX, Icelake-Server-v3, Icelake-Server-v4, Icelake-Server-v5, Icelake-Server-v6, IvyBridge, IvyBridge-IBRS, KnightsMill, Nahalem, Nahalem-IRBS, Opteron_G1, Opteron_G2, Opteron_G3, Opteron_G4, Opteron_G5, Penrym, SandyBridge, SandyBridge-IBRS, SapphireRapids, Skylake-Client, Skylake-Client-IBRS, Skylake-Client-noTSX-IBRS, Skylake-Client-v4, Skylake-Server, Skylake-Server-IBRS, Skylake-Server-noTSX-IBRS, Skylake-Server-v4, Skylake-Server-v5, Westmere, Westmere-IBRS, athlon, core2duo, coreduo, host, kvm32, kvm64, max, pentium, pentium2, pentium3, phenom, qemu32, qemu64, x86-64-v2, x86-64-v2-AES, x86-64-v3, x86-64-v4

==> Wait completed after 14 seconds 92 milliseconds

==> Some builds didn't complete successfully and had errors:
--> ubuntu-cloud-image-2025-10-17.proxmox-iso.ubuntu-cloud-image: Error creating VM: cpuType can only be one of the following values: 486, Broadwell, Broadwell-IBRS, Broadwell-noTSX, Broadwell-noTSX-IBRS, Cascadelake-Server, Cascadelake-Server-V2, Cascadelake-Server-V4, Cascadelake-Server-V5, Cascadelake-Server-noTSX, Conroe, Cooperlake, Cooperlake-V2, EPYC, EPYC-IBPB, EPYC-Milan, EPYC-Rome, EPYC-Rome-v2, EPYC-v3, Haswell, Haswell-IBRS, Haswell-noTSX, Haswell-noTSX-IBRS, Icelake-Client, Icelake-Client-noTSX, Icelake-Server, Icelake-Server-noTSX, Icelake-Server-v3, Icelake-Server-v4, Icelake-Server-v5, Icelake-Server-v6, IvyBridge, IvyBridge-IBRS, KnightsMill, Nahalem, Nahalem-IRBS, Opteron_G1, Opteron_G2, Opteron_G3, Opteron_G4, Opteron_G5, Penrym, SandyBridge, SandyBridge-IBRS, SapphireRapids, Skylake-Client, Skylake-Client-IBRS, Skylake-Client-noTSX-IBRS, Skylake-Client-v4, Skylake-Server, Skylake-Server-IBRS, Skylake-Server-noTSX-IBRS, Skylake-Server-v4, Skylake-Server-v5, Westmere, Westmere-IBRS, athlon, core2duo, coreduo, host, kvm32, kvm64, max, pentium, pentium2, pentium3, phenom, qemu32, qemu64, x86-64-v2, x86-64-v2-AES, x86-64-v3, x86-64-v4