Re: [RFC PATCH 00/18] KVM: VMX: Introduce Intel Mode-Based Execute Control (MBEC)

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 13, 2025, Jon Kohler wrote:
> ## Summary
> This series introduces support for Intel Mode-Based Execute Control
> (MBEC) to KVM and nested VMX virtualization, aiming to significantly
> reduce VMexits and improve performance for Windows guests running with
> Hypervisor-Protected Code Integrity (HVCI).

...

> ## Testing
> Initial testing has been on done on 6.12-based code with:
>   Guests
>     - Windows 11 24H2 26100.2894
>     - Windows Server 2025 24H2 26100.2894
>     - Windows Server 2022 W1H2 20348.825
>   Processors:
>     - Intel Skylake 6154
>     - Intel Sapphire Rapids 6444Y

This series needs testcases, and lots of 'em.  A short list off the top of my head:

 - New KVM-Unit-Test (KUT) ept_access_xxx testcases to verify KVM does the right
   thing with respect to user and supervisor code fetches when MBEC is:

     1. Supported and Enabled
     2. Supported but Disabled
     3. Unsupported

 - KUT testcases to verify VMLAUNCH/VMRESUME consistency checks.

 - KUT testcases to verify KVM treats WRITABLE+USER_EXEC as an illegal combination,
   i.e. that MBEC doesn't affect the W=1,R=0 behavior.

The access tests in particular absolutely need to be provided along with the next
version.  Unless I'm missing something, this RFC implementation is buggy throughout
due to tracking MBEC on a per-vCPU basis, and all of those bugs should be exposed
by even relative basic testcases.