Environment:
Problem:
After installing Windows Updates on a Windows Server 2025 VM configured with cpu: x86-64-v4, the Windows Event Log service crashes immediately on start with exit code 1067. QEMU Guest Agent subsequently fails because it cannot register its event source.
The QEMU Guest Agent error:
critical: unable to register event source: The RPC server is unavailable.
critical: error initializing guest agent
Windows Error Reporting confirmed the root crash:
Fault Module: ntdll.dll (10.0.26100.33438)
Exception Code: 0xC000001D (Illegal Instruction)
Faulting service: svchost.exe_EventLog
Reproduction — Confirmed via A/B testing:
Key finding: cpu: host works on the same physical Xeon 8260 that supports AVX-512 (avx512f, avx512dq, avx512cd, avx512bw, avx512vl, avx512_vnni). This suggests the issue is specific to how QEMU emulates the x86-64-v4 instruction set rather than the physical CPU capabilities.
Steps to reproduce:
Workaround:
Change CPU model to x86-64-v3 or x86-64-v2-AES:
bash
qm set <VMID> --cpu x86-64-v3
This resolves the issue immediately without reinstalling Windows.
Questions:
- Proxmox VE (latest)
- Host CPU: Intel Xeon Platinum 8260 @ 2.40GHz (AVX-512 capable)
- Guest: Windows Server 2025 Datacenter (build 26100.33438)
- Machine type: pc-q35-10.1
- QEMU Guest Agent: 110.0.2
Problem:
After installing Windows Updates on a Windows Server 2025 VM configured with cpu: x86-64-v4, the Windows Event Log service crashes immediately on start with exit code 1067. QEMU Guest Agent subsequently fails because it cannot register its event source.
The QEMU Guest Agent error:
critical: unable to register event source: The RPC server is unavailable.
critical: error initializing guest agent
Windows Error Reporting confirmed the root crash:
Fault Module: ntdll.dll (10.0.26100.33438)
Exception Code: 0xC000001D (Illegal Instruction)
Faulting service: svchost.exe_EventLog
Reproduction — Confirmed via A/B testing:
| State | EventLog | QEMU-GA |
|---|---|---|
| Fresh Windows Server 2025 + x86-64-v4 (no updates) |  Running | Running |
| After Windows Update KB5122871 (Sept 2026, build 26100.33438) + x86-64-v4 |  Crashes (0xC000001D) | Fails |
| x86-64-v2-AES + any updates | Running | Running |
| x86-64-v3 + any updates | Running | Running |
| host + any updates | Running | Running |
Key finding: cpu: host works on the same physical Xeon 8260 that supports AVX-512 (avx512f, avx512dq, avx512cd, avx512bw, avx512vl, avx512_vnni). This suggests the issue is specific to how QEMU emulates the x86-64-v4 instruction set rather than the physical CPU capabilities.
Steps to reproduce:
- Create a Windows Server 2025 VM with cpu: x86-64-v4
- Install Windows Server 2025 fresh
- Verify EventLog and QEMU-GA running
- Run Windows Update — install all updates including KB5122871 (Sept 2026)
- Reboot
- Check EventLog — crashes immediately
Workaround:
Change CPU model to x86-64-v3 or x86-64-v2-AES:
bash
qm set <VMID> --cpu x86-64-v3
This resolves the issue immediately without reinstalling Windows.
Questions:
- Is this a known issue with x86-64-v4 and Windows Server 2025?
- Is x86-64-v4 incorrectly exposing or emulating certain AVX-512 instructions that ntdll.dll in build 26100.33438 uses?
- Should this be reported to Microsoft as well?