CPU Baseline

This page explains how go-pve-qemu picks the --cpu type for new VMs so they can migrate between any nodes in the cluster.

Why a Baseline

Proxmox's host CPU type exposes the current node's full instruction set to the VM; after migrating to a node with an older instruction set the VM fails to boot. Using the lowest level every node supports trades a little performance for unrestricted migration.

Detection Flow

Read the cache file .go_qemu_cpu_type; use it when present
pvesh get /nodes --output-format json to list every node
Run pvesh get /nodes/<node>/status per node and parse cpuinfo.flags
Classify each node with the table below and take the cluster-wide minimum
Write .go_qemu_cpu_type

If any node query fails, detection fails as a whole and qm create falls back to kvm64.

Level Classification

Level	Required flags
`x86-64-v1`	Default when v2 is not met
`x86-64-v2`	`cx16` `lahf_lm` `popcnt` `sse4_1` `sse4_2` `ssse3`
`x86-64-v2-AES`	v2 + `aes` `pclmulqdq` (and v3 not met)
`x86-64-v3`	v2 + `avx` `avx2` `bmi1` `bmi2` `f16c` `fma` `movbe` `lzcnt` `xsaveopt`
`x86-64-v4`	v3 + `avx512f` `avx512bw` `avx512cd` `avx512dq` `avx512vl`

Known Behavior

The cluster-wide minimum only recognizes x86-64-v1 through v4; a node classified as x86-64-v2-AES counts as v1 and drops the whole cluster baseline to x86-64-v1
The cache never expires on its own; delete .go_qemu_cpu_type after adding or replacing nodes
Existing VMs are unaffected; only VMs created afterwards use the new baseline