Windows 11 VM Issues

[refocus_mutt820]

Hi everyone,

In order to avoid the performance penalty of using host (md_clear, flush_l1d), I started using 'x86-64-v4' which works perfectly but I can't pass hypervisor flags to Windows VM. I tried creating custom cpu configuration using( cpu base, EPYC-Genoa, etc..) and add/removing cpu flags, but the performance is still unbearable compared to x86-64-v4.

Setup:
I have a Ryzen 9 9950x3d, 5090, x870e crosshair hero on my Pve 9.0.11 on all the latest updates.

Host( slow performance)

affinity: 0-7,16-23,8-15,24-31
args: -cpu host,-hypervisor,kvm=off, -smbios type=0,vendor="American Megatrends Inc.",version=1715,date="09/19/2025"
balloon: 0
bios: ovmf
boot: order=sata0
cores: 32
cpu: host,hidden=1
efidisk0: local:112/vm-112-disk-0.raw,efitype=4m,pre-enrolled-keys=1,size=528K
hookscript: local:snippets/power-mode-hook.sh
hostpci0: 0000:01:00.0,pcie=1,x-vga=1
hostpci1: 0000:01:00.1,pcie=1
hostpci2: 0000:0d:00,pcie=1
ide2: local:iso/Win11_24H2_English_x64.iso,media=cdrom,size=5683090K
machine: pc-q35-10.1
memory: 32800
meta: creation-qemu=10.0.2,ctime=1757533697
name: Win11
numa: 1
ostype: win11
sata0: local:112/vm-112-disk-1.raw,cache=writeback,size=286G,ssd=1
smbios1: REDACTED
tpmstate0: local:112/vm-112-disk-2.raw,size=4M,version=v2.0
vga: none
vmgenid: REDACTED
root@horsepower:~#

x86-64-v4:

affinity: 0-7,16-23
args: -cpu x86-64-v4,kvm=off,-hypervisor -smbios type=0,vendor="American Megatrends Inc.",version=1715,date="09/19/2025"
balloon: 0
bios: ovmf
boot: order=sata0
cores: 16
cpu: x86-64-v4,hidden=1
efidisk0: local:112/vm-112-disk-0.raw,efitype=4m,pre-enrolled-keys=1,size=528K
hookscript: local:snippets/power-mode-hook.sh
hostpci0: 0000:01:00.0,pcie=1,x-vga=1
hostpci1: 0000:01:00.1,pcie=1
hostpci2: 0000:0d:00,pcie=1
ide0: local:iso/gparted-live-1.7.0-8-amd64.iso,media=cdrom,size=580M
ide2: local:iso/Win11_24H2_English_x64.iso,media=cdrom,size=5683090K
machine: pc-q35-10.1
memory: 32800
meta: creation-qemu=10.0.2,ctime=1757533697
name: Win11
numa: 1
ostype: win11
sata0: local:112/vm-112-disk-1.raw,cache=writeback,size=286G,ssd=1
smbios1: REDACTED
sockets: 1
tpmstate0: local:112/vm-112-disk-2.raw,size=4M,version=v2.0
vga: none
vmgenid: REDACTED

EPYC-Genoa(has no md_clear, flush_l1d but is still slower than x86-64-v4) :

affinity: 0-7,16-23
args: -cpu EPYC-Genoa,-la57,hv_vendor_id=null,-hypervisor -smbios type=0,vendor="American Megatrends Inc.",version=1715,date="09/19/2025"
REMOVED
cores: 16
cpu: EPYC-Genoa,-la57,hidden=1
REMOVED

1. I can't pass in hypervisor/kvm=off flags without Proxmox bootlooping.
2. I can't pass args '-cpu x86-64-v4 ' without Proxmox bootlooping/not starting.
3. I tried starting from cpu 'base' and adding x86-64-v4 cpu flags and some flags aren't found.
4. I tried using qemu64/kvm64 and some flags aren't found.

Any tips?

 

[dakralex]

Hi!

1. I can't pass in hypervisor/kvm=off flags without Proxmox bootlooping.
2. I can't pass args '-cpu x86-64-v4 ' without Proxmox bootlooping/not starting.

x86-64-v4 is not a CPU model configuration that QEMU directly provides, so it only works by specifying it for the qemu-server's cpu config, but not in args.

1. I tried starting from cpu 'base' and adding x86-64-v4 cpu flags and some flags aren't found.
2. I tried using qemu64/kvm64 and some flags aren't found.

What do you mean by "some flags aren't found"? It could be that these features aren't supported by the host CPU itself.

If you want to get the CPU model configuration for x86-64-v4 and disable hypervisor and set kvm=off, then one method is to set the CPU model to x86-64-v4, get the output of qm showcmd <vmid> --pretty | grep "\-cpu" and use that cpu parameter as a baseline, i.e. the following should be the cpu parameter. Does that solve the problem for you?

-cpu 'qemu64,+abm,+aes,+avx,+avx2,+avx512bw,+avx512cd,+avx512dq,+avx512f,+avx512vl,+bmi1,+bmi2,enforce,+f16c,+fma,hv_ipi,hv_relaxed,hv_reset,hv_runtime,hv_spinlocks=0x1fff,hv_stimer,hv_synic,hv_time,hv_vapic,hv_vpindex,+kvm_pv_eoi,+kvm_pv_unhalt,+movbe,+pni,+popcnt,+sse4.1,+sse4.2,+ssse3,+xsave,-hypervisor,kvm=off'