Upgrading from x86_64-v2

[Dwid]

Hi, what are the consequences if I change the CPU architecture of a VM from x86_64-v2 to x86_64-v3?
From my testing I reall can't see any issues, everything seems to be running fine. But I guess given the changes that Redhat have introduces with RHEL 10 and setting x86_64-v3 as a requirement for upgrading it seems relevant to ask the question.
I would assume that it would prove problematic going from x86_64-v3 to x86_64-v2 and not the other way.
Anyone who might shed some light on this, and maybe have some experience in this?

Dwid.

 

[bbgeek17]

You can find functional differences between CPU feature sets across x86-64 microarchitectures here: x86-64 Feature Levels – Wikipedia

Changing the CPU type in a VM requires a cold start (full power-off and restart) for the new settings to take effect.

Most modern operating systems, including Windows, handle such changes without issue. However, some software (including OS licensing mechanisms) may tie activation to a hardware fingerprint, which could be affected by the CPU change.

Take a snapshot of the VM before testing any changes.

Good luck

---

Blockbridge : Ultra low latency all-NVME shared storage for Proxmox - https://www.blockbridge.com/proxmox

 

[SteveITS]

Adding to the above reply, we haven't noticed anything when changing CPU types.

There's a CPU compatibility list at https://qemu-project.gitlab.io/qemu/system/qemu-cpu-models.html

 

[ucholak]

• app can be compiled to use faster cpu instructions (ex: avx512), if available and detected at runtime. bud it needs to see it.
  ex: ffmpeg, mplayer (./configure --enable-runtime-cpudetection).

• gcc $CFLAGS can be configured with -march=native
  app speed will change after app recompilation.
  then you cant downgrade cpu, program will not run (ilegal cpu instruction).
  ex: gentoo

• gcc $CFLAGS can be configured with -mtune=native,
  app speed will change after app recompilation.
  ex: gentoo

• gcc $CFLAGS can be configured with -mtune=x86_64-v4,
  app will be running fastest only on this cpu arch
  ex: cachyOS

• IHMO you should use highest possible feature level for your cluster, and lowest common cpu denominator (all cpus have this feature set)

• x86_64-v2 is just safe default in last proxmox release, ok with cpus decade+ old, reasonable fast instructions (sse4), and probably required to solve default windows 11 install requirements