If you configure multiple IOthreads, ex:

        ioThreadsPolicy: supplementalPool
        ioThreads:
          supplementalPoolThreadCount: 4

and the VM definition includes:
isolateEmulatorThread: true

the VM will fail to start with an error like:

{"component":"virt-launcher","level":"error","msg":"cannot set CPU affinity on process 844: Invalid argument","pos":"virProcessSetAffinity:550","subcomponent":"libvirt","thread":"28","timestamp":"2025-06-23T14:51:41.816000Z"}

4.19

always

1. Use highperformance template VM
2. Add supplementalPool & supplementalPoolThreadCount > 1

VM fails to start

At the least should probably warn / prevent this in a VM config validation check for now?

Otherwise a "full" solution will take more work to pin IOthreads to additional host cpus beyond the single emulator thread...