Qemu 5 on Windows Host With WHPX

Running Linux guests using Qemu 5.0+ on a Windows host, using the WHPX accelerator, may fail to start and freezes on this line:

[ ... ] Freeing unused kernel image memory: ...

These guests work fine using Qemu 4 on the same Windows host, configured to use a SkyLake CPU type. The issue can be worked around in either of two ways:

  1. Switch the accelerator to TCG.
    Enable SMP with a suitable number of cores or threads to improve performance. I.e. use the parameters:
    -accel tcg -cpu Skylake-Client -smp sockets=1,cores=2,threads=2

  2. Switch the CPU type to Westmere.
    We have confirmed the guest fails to boot with a CPU type of SandyBridge or later. You can also enable the pcid on the CPU to mitigate the cost of Meltdown fixes. I.e. use the parameters:
    -accel whpx -cpu Westmere,+pcid

Running Windows guests with the WHPX accelerator and SMP enabled may cause the guest to freeze, with the host at 100% high CPU usage, after running for a short while. To prevent this, either:

  1. Remove the “-smp …” parameter.
    – or –
  2. Limit to 1 CPU, 1 core and 1 thread, i.e.:
    -smp sockets=1,cores=1,threads=1

This will limit Qemu to single core on the host, but performance will still be better than using the TCG accelerator.

Also for Windows VMs’ be sure to add the CPU enlightenments to signal Windows guests to use optimizations for running in a virtual environment:

-cpu Skylake-Client-IBRS,hv_crash,hv_frequencies,hv_relaxed,hv_reset,hv_runtime,
hv_spinlocks=0x1fff,hv_time,hv_vapic

For detailed information see:
https://github.com/qemu/qemu/blob/master/docs/hyperv.txt