Windows 11 24H2 (KB5063060) fails to boot with 'UNSUPPORTED PROCESSOR' on multi-core QEMU guests with AMD EPYC host

Host environment

• Operating system: Debian Trixie
• OS/kernel version: 6.12.27-amd64
• Architecture: x86
• QEMU flavor: qemu-system-x86_64
• QEMU version: 10.0.0 (Debian 1:10.0.0+ds-2)
• QEMU command line:

This VM is managed via an infrastructure as code tool - below is the automatically generated command from libvirt -

LC_ALL=C \
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin \
USER=root \
HOME=/var/lib/libvirt/qemu/domain-255-win11-testvm \
XDG_DATA_HOME=/var/lib/libvirt/qemu/domain-255-win11-testvm/.local/share \
XDG_CACHE_HOME=/var/lib/libvirt/qemu/domain-255-win11-testvm/.cache \
XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain-255-win11-testvm/.config \
SPICE_DEBUG_ALLOW_MC=1 \
/usr/bin/kvm \
-name guest=win11-testvmernal,debug-threads=on \
-S \
-object '{"qom-type":"secret","id":"masterKey0","format":"raw","file":"/var/lib/libvirt/qemu/domain-255-win11-testvm/master-key.aes"}' \
-blockdev '{"driver":"file","filename":"/usr/share/OVMF/OVMF_CODE_4M.ms.fd","node-name":"libvirt-pflash0-storage","auto-read-only":true,"discard":"unmap"}' \
-blockdev '{"node-name":"libvirt-pflash0-format","read-only":true,"driver":"raw","file":"libvirt-pflash0-storage"}' \
-blockdev '{"driver":"file","filename":"/var/lib/libvirt/qemu/nvram/win11-testvmernal_VARS.fd","node-name":"libvirt-pflash1-storage","read-only":false}' \
-machine pc-q35-9.0,usb=off,vmport=off,smm=on,dump-guest-core=off,memory-backend=pc.ram,pflash0=libvirt-pflash0-format,pflash1=libvirt-pflash1-storage,graphics=off,hpet=off,acpi=on \
-accel kvm \
-cpu EPYC-Genoa,x2apic=on,tsc-deadline=on,hypervisor=on,tsc-adjust=on,spec-ctrl=on,stibp=on,flush-l1d=on,arch-capabilities=on,ssbd=on,cmp-legacy=on,overflow-recov=on,succor=on,virt-ssbd=on,lbrv=on,tsc-scale=on,vmcb-clean=on,flushbyasid=on,pause-filter=on,pfthreshold=on,v-vmsave-vmload=on,vgif=on,sbpb=on,ibpb-brtype=on,perfmon-v2=on,rdctl-no=on,skip-l1dfl-vmentry=on,mds-no=on,pschange-mc-no=on,gds-no=on,rfds-no=on,hv-time=on,hv-relaxed=on,hv-vapic=on,hv-spinlocks=0x1fff \
-global driver=cfi.pflash01,property=secure,value=on \
-m size=16777216k \
-object '{"qom-type":"memory-backend-ram","id":"pc.ram","size":17179869184}' \
-overcommit mem-lock=off \
-smp 4,sockets=1,dies=1,clusters=1,cores=4,threads=1 \
-uuid 569c7924-3cdb-5c9e-bd0f-bff6582512cf \
-no-user-config \
-nodefaults \
-chardev socket,id=charmonitor,fd=243,server=on,wait=off \
-mon chardev=charmonitor,id=monitor,mode=control \
-rtc base=utc,driftfix=slew \
-global kvm-pit.lost_tick_policy=delay \
-no-shutdown \
-global ICH9-LPC.disable_s3=1 \
-global ICH9-LPC.disable_s4=1 \
-boot menu=on,strict=on \
-device '{"driver":"pcie-root-port","port":16,"chassis":1,"id":"pci.1","bus":"pcie.0","multifunction":true,"addr":"0x2"}' \
-device '{"driver":"pcie-root-port","port":17,"chassis":2,"id":"pci.2","bus":"pcie.0","addr":"0x2.0x1"}' \
-device '{"driver":"pcie-root-port","port":18,"chassis":3,"id":"pci.3","bus":"pcie.0","addr":"0x2.0x2"}' \
-device '{"driver":"pcie-root-port","port":19,"chassis":4,"id":"pci.4","bus":"pcie.0","addr":"0x2.0x3"}' \
-device '{"driver":"pcie-root-port","port":20,"chassis":5,"id":"pci.5","bus":"pcie.0","addr":"0x2.0x4"}' \
-device '{"driver":"qemu-xhci","id":"usb","bus":"pci.3","addr":"0x0"}' \
-device '{"driver":"virtio-scsi-pci","id":"scsi0","bus":"pci.2","addr":"0x0"}' \
-device '{"driver":"virtio-serial-pci","id":"virtio-serial0","bus":"pci.4","addr":"0x0"}' \
-blockdev '{"driver":"file","filename":"/srv/vm/win11-testvm.raw","node-name":"libvirt-2-storage","read-only":false,"discard":"unmap","cache":{"direct":false,"no-flush":false}}' \
-device '{"driver":"scsi-hd","bus":"scsi0.0","channel":0,"scsi-id":0,"lun":0,"device_id":"drive-scsi0-0-0-0","drive":"libvirt-2-storage","id":"scsi0-0-0-0","bootindex":1,"write-cache":"on"}' \
-device '{"driver":"scsi-cd","bus":"scsi0.0","channel":0,"scsi-id":0,"lun":1,"device_id":"drive-scsi0-0-0-1","id":"scsi0-0-0-1","bootindex":2,"write-cache":"on"}' \
-netdev '{"type":"tap","fd":"273","vhost":true,"vhostfd":"304","id":"hostnet0"}' \
-device '{"driver":"virtio-net-pci","netdev":"hostnet0","id":"net0","mac":"5c:40:b0:48:86:d6","bus":"pci.1","addr":"0x0"}' \
-chardev pty,id=charserial0 \
-device '{"driver":"isa-serial","chardev":"charserial0","id":"serial0","index":0}' \
-chardev qemu-vdagent,id=charchannel0,name=vdagent,clipboard=on \
-device '{"driver":"virtserialport","bus":"virtio-serial0.0","nr":1,"chardev":"charchannel0","id":"channel0","name":"com.redhat.spice.0"}' \
-chardev socket,id=charchannel1,fd=238,server=on,wait=off \
-device '{"driver":"virtserialport","bus":"virtio-serial0.0","nr":2,"chardev":"charchannel1","id":"channel1","name":"org.qemu.guest_agent.0"}' \
-chardev socket,id=chrtpm,path=/run/libvirt/qemu/swtpm/255-win11-testvm-swtpm.sock \
-tpmdev emulator,id=tpm-tpm0,chardev=chrtpm \
-device '{"driver":"tpm-crb","tpmdev":"tpm-tpm0","id":"tpm0"}' \
-device '{"driver":"usb-tablet","id":"input0","bus":"usb.0","port":"1"}' \
-device '{"driver":"usb-mouse","id":"input1","bus":"usb.0","port":"2"}' \
-device '{"driver":"usb-kbd","id":"input2","bus":"usb.0","port":"3"}' \
-audiodev '{"id":"audio1","driver":"none"}' \
-vnc vnc=unix:/var/lib/libvirt/qemu/domain-255-win11-testvm/vnc.sock,audiodev=audio1 \
-device '{"driver":"qxl-vga","id":"video0","max_outputs":1,"ram_size":67108864,"vram_size":67108864,"vram64_size_mb":0,"vgamem_mb":16,"bus":"pcie.0","addr":"0x1"}' \
-device '{"driver":"ich9-intel-hda","id":"sound0","bus":"pcie.0","addr":"0x1b"}' \
-device '{"driver":"hda-duplex","id":"sound0-codec0","bus":"sound0.0","cad":0,"audiodev":"audio1"}' \
-global ICH9-LPC.noreboot=off \
-watchdog-action reset \
-device '{"driver":"virtio-balloon-pci","id":"balloon0","bus":"pcie.0","addr":"0x8"}' \
-sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny \
-msg timestamp=on

Emulated/Virtualized environment

• Operating system: Windows 11
• OS/kernel version: 24H2
• Architecture: x86

Description of problem

The Windows 11 cumulative update from 09 June 2025, KB5063060, causes QEMU VMs running on AMD EPYC Genoa hosts to fail to boot with an “UNSUPPORTED PROCESSOR” error when configured with multiple vCPUs. This update supersedes KB5060842, which exhibited the same problem.

Our hardware uses dual AMD EPYC 9374F CPUs. Similar reports have appeared on community forums since the update's release, suggesting this probably impacts other recent AMD EPYC CPUs.

Is it possible that some CPU features are either not fully supported or are being exposed to Windows incorrectly, and this recent KB update is just a symptom?

• Workaround: Block KB5063060 from installing or drop all Windows 11 VMs to one core.

Steps to reproduce

1. On an AMD EPYC hypervisor, spin up a Windows 11 VM with multiple cores and whatever other hardware and install KB5063060.
2. Upon reboot, note the ‘UNSUPPORTED PROCESSOR’ blue screen / error. The system will fail to boot.
3. Drop the VM’s core count to 1 OR try to remove the update from startup repair, and the VM will resume booting fine again.

Additional information

Windows startup repair references SrtTrail.txt, but boot seems to fail before it is populated. I will add logs to the report if I can get any to generate.

See XML attached. win11-test.xml