Comment 0 for bug 1911848

[Impact]

On CascadeLake based KVM hosts, Windows Server 2k16 and 2k19 guests will fail to start once they have enabled the hyper-v role for nested virtualisation.

The Windows Server guests will get stuck in the late stages of boot, before the graphical login screen appears, on Windows Server systems with the desktop environment installed.

If you look at performance metrics for the guest, the CPU will appear to be stuck at 100%, and it never changes from 100%. The Windows Server guest is unresponsive.

The KVM settings use Cascadelake-Server-noTSX virtual CPUs, with some very specific settings needed for nested virtualisation. See testcase section. If you use any other vcpu type, the problem does not reproduce.

Known workarounds are to install the 5.8 HWE kernel, in which case the server will come up as expected.

[Fix]

The following commit fixes the issue, and landed in mainline 5.8-rc1:

commit 8081ad06b68a728e676d3b08e9ab70ce4039747b
Author: Sean Christopherson <email address hidden>
Date: Wed Apr 22 19:25:40 2020 -0700
Subject: KVM: x86: Set KVM_REQ_EVENT if run is canceled with req_immediate_exit set
Link: https://github.com/torvalds/linux/commit/8081ad06b68a728e676d3b08e9ab70ce4039747b

It appears that pending requests to the hypervisor can be lost or delayed if an immediate exit was requested in vcpu_enter_guest(). As the commit message mentions, only the !injected case is affected, so we add a check at the cancel_injection label to see if we got there as a result of an immediate exit, and then re-issue a KVM_REQ_EVENT request if we are.

The Windows guest is waiting for an event to be processed, which never happens, and so gets stuck.

Even though the above commit has a Fixes: tag to a commit in 3.15-rc1, in my testing the 4.15 kernel with a Bionic-ussuri userspace does not reproduce the issue, so SRU to Bionic will not be needed.

[Testcase]

A cascadelake based Xeon server is required. Anything else and the bug will not reproduce.

I used a c5.metal server on AWS. It has the following processor:
Intel(R) Xeon(R) Platinum 8275CL CPU @ 3.00GHz

Install a KVM stack, and ubuntu-desktop. Set up xrdp and confirm you can reach the desktop. Copy a Windows Server 2k19 image to the destination server, as well as a recent ISO image of virtio drivers.

Install virt-manager.

Create a new virtual machine using the Windows 2k19 defaults. Use 8 vcpus, 16gb ram. Click customise button to change settings before install.

Change the hard disk to be SATA, attach a new cd rom driver for the virtio drivers. Change networking to virtio. Change CPU to Cascadelake-Server-noTSX.

Edit the virsh xml, and ensure you set the following features for CPU:

  <cpu mode='custom' match='exact' check='full'>
    <model fallback='forbid'>Cascadelake-Server-noTSX</model>
    <topology sockets='8' cores='1' threads='1'/>
    <feature policy='require' name='invpcid'/>
    <feature policy='require' name='pcid'/>
    <feature policy='require' name='vmx'/>
    <feature policy='require' name='hypervisor'/>
    <feature policy='disable' name='mpx'/>
    <feature policy='require' name='pku'/>
    <feature policy='require' name='arch-capabilities'/>
    <feature policy='require' name='rdctl-no'/>
    <feature policy='require' name='ibrs-all'/>
    <feature policy='require' name='skip-l1dfl-vmentry'/>
    <feature policy='require' name='mds-no'/>
  </cpu>

Those settings are an absolute must.

Boot the VM, and install Windows 2k19 with the desktop environment. Once it is installed, open up computer management > device manager and install drivers from the virtio ISO for missing hardware, likely the network and balloon devices.

From there, go to server manager, and install the hyper-v role.

Reboot the server. It will reboot a few times, and on the final time, it will lock up before it reaches the log in screen.

In virt-manager, go to the performance tab. The CPU will be stuck at 100%. The windows guest will be non responsive.

A patched kernel is available in the following ppa:

https://launchpad.net/~mruffell/+archive/ubuntu/sf296306-test

If you install this kernel and boot the Windows 2k19 guest, it will come up normally when the hyper-v role is enabled, and you will be able to log in.

[Where problems could occur]

This is a change to a core part of the kvm subsystem, so there is potential for regression which could affect all users of KVM.

If a regression were to occur, there are no workarounds. Users would need to downgrade their kernel while a fix is developed.