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How To Fix “Spectre Mitigation/lfence not serializing” Error on Ubuntu 18.04.1 in VirtualBox

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In this article, we will discuss how to fix the “Spectre Mitigation/lfence not serializing” error that can occur on Ubuntu 18.04.1 when running in VirtualBox. This error is related to the Spectre v2 vulnerability and can cause issues during the startup of your Ubuntu Virtual Machine (VM).

Understanding the Error

“Spectre Mitigation/lfence not serializing” is a warning message that appears during the boot process of an Ubuntu VM in VirtualBox. It is related to the Spectre v2 vulnerability, a hardware vulnerability affecting microprocessors that perform branch prediction. The warning message indicates that the mitigation technique used to prevent the Spectre v2 vulnerability is not working as expected.

Possible Causes

There are several possible causes for this error. One is the presence of older GPU hardware. However, if you’re running a newer GPU, such as an Nvidia GTX 1060, this is unlikely to be the cause.

Another possible cause is the automatic enabling of Hyper-V by Docker on a Windows 10 host. Docker is a platform used for developing, shipping, and running applications. Hyper-V is a virtualization technology from Microsoft. When Docker automatically enables Hyper-V, it can affect the behavior of the Ubuntu guest OS in VirtualBox.

Solutions

Disabling Hyper-V

If Docker’s automatic enabling of Hyper-V is causing the issue, you can disable Hyper-V in the Windows 10 host settings. To do this:

  1. Press Win+R to open the Run dialog.
  2. Type “OptionalFeatures.exe” and press Enter.
  3. In the Windows Features dialog that appears, uncheck the Hyper-V option.
  4. Click OK to apply the changes and close the dialog.

Enabling an Additional Core in VirtualBox

Another possible solution is to enable an additional core in VirtualBox settings. This might help in getting past the error message during boot. To do this:

  1. Open VirtualBox.
  2. Select your Ubuntu VM and click on Settings.
  3. Go to System -> Processor.
  4. Increase the number of CPUs to 2 or more.
  5. Click OK to apply the changes and close the dialog.

Using VirtualBox as the Docker Engine

If you’re using Docker, another solution is to configure Docker to use VirtualBox as the Docker engine instead of Hyper-V. This can potentially resolve the issue. To do this:

  1. Open Docker.
  2. Go to Settings -> General.
  3. Uncheck the “Use the WSL 2 based engine” option.
  4. Click Apply & Restart to apply the changes and restart Docker.

Uninstalling Docker

If Docker keeps re-enabling Hyper-V after each restart, it may be necessary to uninstall Docker. This can prevent Docker from automatically enabling Hyper-V and potentially interfering with the Ubuntu VM startup. To uninstall Docker:

  1. Open the Control Panel.
  2. Go to Programs -> Programs and Features.
  3. Select Docker from the list and click Uninstall.

Conclusion

The “Spectre Mitigation/lfence not serializing” error during the startup of an Ubuntu 18.04.1 VM in VirtualBox can be caused by various factors. By disabling Hyper-V in the Windows 10 host settings, enabling an additional core in VirtualBox, using VirtualBox as the Docker engine, or uninstalling Docker, you can potentially resolve the issue.

What is the Spectre v2 vulnerability?

The Spectre v2 vulnerability is a hardware vulnerability that affects microprocessors. It involves exploiting a weakness in branch prediction, a feature of microprocessors that allows them to predict which instructions will be executed next.

How does the “Spectre Mitigation/lfence not serializing” error impact my Ubuntu VM?

The "Spectre Mitigation/lfence not serializing" error is a warning message that appears during the boot process of an Ubuntu VM in VirtualBox. While it does not necessarily cause any immediate issues, it indicates that the mitigation technique used to prevent the Spectre v2 vulnerability is not working properly.

Can older GPU hardware cause the “Spectre Mitigation/lfence not serializing” error?

It is possible that older GPU hardware may cause the error. However, if you are running a newer GPU, such as an Nvidia GTX 1060, it is unlikely to be the cause.

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