kvm: Windows 2k19 with Hyper-v role gets stuck on pending hypervisor requests on cascadelake based kvm hosts

Bug #1911848 reported by Matthew Ruffell on 2021-01-15
This bug affects 1 person
Affects Status Importance Assigned to Milestone
linux (Ubuntu)
Matthew Ruffell

Bug Description

BugLink: https://bugs.launchpad.net/bugs/1911848


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.


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.


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 drive 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'/>

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:


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.

CVE References

Changed in linux (Ubuntu):
status: New → Fix Released
Changed in linux (Ubuntu Focal):
status: New → In Progress
importance: Undecided → Medium
assignee: nobody → Matthew Ruffell (mruffell)
tags: added: focal sts
description: updated
description: updated
Changed in linux (Ubuntu Focal):
status: In Progress → Fix Committed

This bug is awaiting verification that the kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-focal' to 'verification-done-focal'. If the problem still exists, change the tag 'verification-needed-focal' to 'verification-failed-focal'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

tags: added: verification-needed-focal
Matthew Ruffell (mruffell) wrote :

Attached is reproducing the problem with 5.4.0-65-generic, note, see how CPU reaches 100% and is stuck.

Matthew Ruffell (mruffell) wrote :

Attached is reproducing the problem with 5.4.0-66-generic. Note that CPU is low, and system boots to desktop correctly.

Matthew Ruffell (mruffell) wrote :

Performing verification for Focal.

I installed 5.4.0-65-generic to a c5.metal AWS machine, and installed ubuntu-desktop and a KVM stack. I install xrdp and verified I could reach the VMs desktop.

I installed virt-manager, and configured the Windows 2k19 VM in accordance with the testcase section, carefully making sure the Cascadelake-Server-noTSX processor is selected and the features in place.

I installed Windows, and installed the virtio drivers. From there I enabled Hyper-V and restarted.

The machine got stuck during reboot, after the hyper-v feature had installed and configured. Looking at the performance tab, the machine is stuck at 100% CPU, see 5.4.0-65-generic.png attached.

I then enabled -proposed and installed 5.4.0-66-generic, and kexec rebooted. From there I started the windows VM, which came up properly to the desktop. Looking at the performance tab, CPU usage is low, and is what is expected. See 5.4.0-66-generic.png attached.

The 5.4.0-66-generic kernel fixes the issue, happy to mark as verified.

tags: added: verification-done-focal
removed: verification-needed-focal
Launchpad Janitor (janitor) wrote :
Download full text (60.8 KiB)

This bug was fixed in the package linux - 5.4.0-66.74

linux (5.4.0-66.74) focal; urgency=medium

  * focal/linux: 5.4.0-66.74 -proposed tracker (LP: #1913152)

  * Add support for selective build of special drivers (LP: #1912789)
    - [Packaging] Add support for ODM drivers
    - [Packaging] Turn on ODM support for amd64

  * Packaging resync (LP: #1786013)
    - update dkms package versions
    - update dkms package versions

  * Introduce the new NVIDIA 460-server series and update the 460 series
    (LP: #1913200)
    - [Config] dkms-versions -- drop NVIDIA 435 455 and 440-server
    - [Config] dkms-versions -- add the 460-server nvidia driver

  * Enable mute and micmute LED on HP EliteBook 850 G7 (LP: #1910102)
    - ALSA: hda/realtek: Enable mute and micmute LED on HP EliteBook 850 G7

  * SYNA30B4:00 06CB:CE09 Mouse on HP EliteBook 850 G7 not working at all
    (LP: #1908992)
    - HID: multitouch: Enable multi-input for Synaptics pointstick/touchpad device

  * HD Audio Device PCI ID for the Intel Cometlake-R platform (LP: #1912427)
    - SAUCE: ALSA: hda: Add Cometlake-R PCI ID

  * switch to an autogenerated nvidia series based core via dkms-versions
    (LP: #1912803)
    - [Packaging] nvidia -- use dkms-versions to define versions built
    - [Packaging] update-version-dkms -- maintain flags fields
    - [Config] dkms-versions -- add transitional/skip information for nvidia

  * udpgro.sh in net from ubuntu_kernel_selftests seems not reflecting sub-test
    result (LP: #1908499)
    - selftests: fix the return value for UDP GRO test

  * qede: Kubernetes Internal DNS Failure due to QL41xxx NIC not supporting IPIP
    tx csum offload (LP: #1909062)
    - qede: fix offload for IPIP tunnel packets

  * Use DCPD to control HP DreamColor panel (LP: #1911001)
    - SAUCE: drm/dp: Another HP DreamColor panel brigntness fix

  * kvm: Windows 2k19 with Hyper-v role gets stuck on pending hypervisor
    requests on cascadelake based kvm hosts (LP: #1911848)
    - KVM: x86: Set KVM_REQ_EVENT if run is canceled with req_immediate_exit set

  * Ubuntu 20.10 four needed fixes to 'Add driver for Mellanox Connect-IB
    adapters' (LP: #1905574)
    - net/mlx5: Fix a race when moving command interface to polling mode

  * Fix right sounds and mute/micmute LEDs for HP ZBook Fury 15/17 G7 Mobile
    Workstation (LP: #1910561)
    - ALSA: hda/realtek: fix right sounds and mute/micmute LEDs for HP machines

  * Ubuntu 20.04 - multicast counter is not increased in ip -s (LP: #1901842)
    - net/mlx5e: Fix multicast counter not up-to-date in "ip -s"

  * eeh-basic.sh in powerpc from ubuntu_kernel_selftests timeout with 5.4 P8 /
    P9 (LP: #1882503)
    - selftests/powerpc/eeh: disable kselftest timeout setting for eeh-basic

  * DMI entry syntax fix for Pegatron / ByteSpeed C15B (LP: #1910639)
    - Input: i8042 - unbreak Pegatron C15B

  * CVE-2020-29372
    - mm: check that mm is still valid in madvise()

  * update ENA driver, incl. new ethtool stats (LP: #1910291)
    - net: ena: Change WARN_ON expression in ena_del_napi_in_range()
    - net: ena: ethtool: convert stat_offset to 64 bit resolution
    - net: ena: eth...

Changed in linux (Ubuntu Focal):
status: Fix Committed → Fix Released
To post a comment you must log in.
This report contains Public information  Edit
Everyone can see this information.

Other bug subscribers