Guest CPU topology vs pin map

The action in this article has been conducted under the following conditions:

Property	Value
Date	September 2026
Kernel	6.8.0-139-generic
Distribution	Linux Mint 22.3
libvirt	10.0.0

Problem

<cputune> says which host CPU each vCPU thread runs on. <cpu><topology> is what the guest OS believes.

Those two must describe the same shape. If they do not, Windows schedules as if some vCPUs were SMT siblings when they are not.

A common first pin on a 16-thread AMD host is:

<vcpu placement="static" cpuset="8-15">8</vcpu>

with one vCPU per host CPU 8 through 15. On Zen, those numbers are usually the second thread of eight different physical cores (0/8, 1/9, …), not four cores with two threads each.

Advertising this to the guest is then wrong:

<topology sockets="1" dies="1" cores="4" threads="2"/>

Windows will treat vCPU 0+1, 2+3, … as siblings. They are not.

Approach

Keep the host pin list (8-15 or whatever you already chose). Change only the advertised topology so each vCPU is a full core with one thread:

<cpu mode="host-passthrough" check="none" migratable="on">
  <topology sockets="1" dies="1" cores="8" threads="1"/>
  <cache mode="passthrough"/>
  <feature policy="require" name="topoext"/>
</cpu>

topoext stays on so the guest still sees AMD topology extensions. This does not move QEMU threads and does not change hugepages, governors, or slice affinity.

Do not switch to cores="4" threads="2" unless the pin list is real SMT pairs, for example 4,12,5,13,6,14,7,15 on a 3700X CCD. That is a different layout.

Apply

The running domain keeps the old topology until the guest is shut down and started again. virsh define updates the persistent XML immediately.

virsh dumpxml --inactive <domain> > /tmp/<domain>.xml

Edit the <topology> line, then:

virsh define /tmp/<domain>.xml

Confirm:

virsh dumpxml --inactive <domain> | grep -A4 '<cpu '

After the next guest power cycle:

virsh dumpxml <domain> | grep topology

Inside Windows, Task Manager should show 8 cores / 8 logical processors, not 4 cores / 8 logical processors.

Notes

Same logical CPU count (8) so this is not a vCPU resize.
Leave <vcpupin> as-is when only fixing the advertised shape.
migratable="on" is unrelated; it can stay.