Bun segfaults on Proxmox VM — the $6.75 AVX rabbit hole

bun-avx-proxmox.md

Bun Segfaults on Proxmox VM — the $6.75 AVX Rabbit Hole

2026-03-03 — debugging war story

The Problem

Deploying a Bun app to a Proxmox VM. Everything works locally. On the VM:

panic(main thread): Segmentation fault at address 0x419E4000000
CPU lacks AVX support. Please consider upgrading to a newer CPU.

RSS: 15.96 GB. Peak: 7.93 GB. Bun's JIT compiler tried to eat the whole VM alive before dying.

What I Tried (the expensive part)

1. Bundled the app into a single 8MB JS file → same crash, 16GB RSS
2. Ran from source with bun --smol → hangs, no output, then segfault
3. Increased VM RAM from 4GB to 8GB → same crash, just took longer
4. Tried --compile standalone binary → still needs AVX at runtime
5. Considered rewriting the server to run on Node.js instead of Bun

~15-20 rounds of SSH commands, reading crash logs, trying flags. Claude Opus context window growing fatter each round.

Estimated burn: ~300K input tokens + ~30K output tokens = ~$6.75 USD.

The Fix

# Check host CPU — AVX is there
grep -o 'avx[^ ]*' /proc/cpuinfo | head -2
# avx
# avx2

# Check VM config — no CPU type set (defaults to kvm64, no AVX passthrough)
qm config 102 | grep cpu
# (empty)

# The $6.75 command
qm set 102 --cpu host
qm stop 102 && qm start 102

One command. Proxmox defaults VM CPU to kvm64 which strips out AVX/AVX2/SSE4.2 — doesn't pass through host CPU features. Setting --cpu host passes everything through.

After restart:

$ grep -o 'avx[^ ]*' /proc/cpuinfo | head -2
avx
avx2

$ bun --version
1.3.10

$ systemctl --user status my-app
● Active: active (running)
  Memory: 52.2M

52 MB. Not 16 GB. Works perfectly.

Lesson

When your runtime segfaults on a VM with "CPU lacks AVX support" — don't debug the runtime. Check qm config first.

Proxmox CPU types that pass AVX:

• host — pass through all host CPU features (easiest, use this for dedicated VMs)
• x86-64-v3 — guarantees AVX/AVX2 (better for migration between hosts)

Proxmox CPU types that DON'T have AVX:

• kvm64 (default) — minimal feature set
• qemu64 — also minimal
• x86-64-v2 — SSE4.2 yes, AVX no

Runtimes That Need AVX

Runtime	AVX Required	Notes
Bun	Yes	JIT compiler uses AVX. Segfaults without it.
Node.js	No	Works on kvm64. V8 has non-AVX fallbacks.
Deno	No	V8-based, same as Node.

If you can't change the VM CPU type, use Node.js. If you control the hypervisor, just set --cpu host and move on.

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$6.75 in AI tokens to learn one qm set command. At least now you don't have to.