EC2, Windows 10, half of the virtual processors

This issue happens when Hyper-Threading is disabled at OS level. This may have been done based on Microsoft recommendation for protecting against a known vulnerability known as speculative execution side-channel [1].

Based on "Windows guidance to protect against speculative execution side-channel vulnerabilities"[2] that Customers who have Windows Update enabled and have applied the security updates released on July 9, 2019 are protected automatically. There is no further configuration necessary.

To resolve this issue, I suggested enabling Hyper-Threading using the below steps:

1. Confirm that Hyper-Threading is disabled by querying the registry using below command:

    •   reg query "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management" /v FeatureSettingsOverride 

    The Hyper-Threading will be disabled if the output is:

    •   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management
    •   FeatureSettingsOverride    REG_DWORD    0x2048

2. Once confirmed, proceed with enabling the Hyper-Threading using the below command:

    •   reg add "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management" /v FeatureSettingsOverride /t REG_DWORD /d 0 /f

3. Restart the instance and check the number of vCPU reported in the OS, which should show the correct number.

Reference:

• [1] Microsoft Speculative Execution Side-Channel Vulnerability: https://support.microsoft.com/en-us/help/4457951/windows-guidance-to-protect-against-speculative-execution-side-channel
• [2] Microsoft Mitigation Steps: https://support.microsoft.com/en-us/help/4072698/windows-server-speculative-execution-side-channel-vulnerabilities