Re: [PATCH V2 4/4] KVM: selftests: Add nested page fault injection test

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 24, 2026, Kevin Cheng wrote:
> Add a test that exercises nested page fault injection during L2
> execution. L2 executes I/O string instructions (OUTSB/INSB) that access
> memory restricted in L1's nested page tables (NPT/EPT), triggering a
> nested page fault that L0 must inject to L1.
> 
> The test supports both AMD SVM (NPF) and Intel VMX (EPT violation) and
> verifies that:
>   - The exit reason is an NPF/EPT violation
>   - The access type and permission bits are correct
>   - The faulting GPA is correct
> 
> Three test cases are implemented:
>   - Unmap the final data page (final translation fault, OUTSB read)
>   - Unmap a PT page (page walk fault, OUTSB read)
>   - Write-protect the final data page (protection violation, INSB write)
>   - Write-protect a PT page (protection violation on A/D update, OUTSB
>     read)

Either in this test or in KUT, we need coverage for validating faults that are
reported by hardware, i.e. for faults that _don't_ go through the emulator.

E.g. there's this "todo" of sorts in KUT:

	case VMX_EPT_VIOLATION:
		/*
		 * Exit-qualifications are masked not to account for advanced
		 * VM-exit information. Once KVM supports this feature, this
		 * masking should be removed.
		 */
		exit_qual &= ~EPT_VLT_GUEST_MASK;


Or maybe both?  I generally prefer selftests for maintenance purposes, and you've
already written this test...