Re: [RFC PATCH 4/5] KVM: selftests: nVMX: Add nested PML coverage to nested_dirty_log_test

["Nikunj A. Dadhania" <nikunj@amd.com>]

On 10/15/2026 4:34 PM, sashiko-bot@kernel.org wrote:
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> 
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] selftests: kvm: nVMX test asserts on invalid guest PML index
> --
> 
> commit 88c8168ec493a327cfe182178b11f281733406e7
> Author: Nikunj A Dadhania <nikunj@amd.com>
> 
> KVM: selftests: nVMX: Add nested PML coverage to nested_dirty_log_test
> 
> This patch adds coverage for nested PML to nested_dirty_log_test. It
> verifies that L1 takes a PML_FULL exit when the buffer fills, can drain it,
> and can resume L2 without losing entries.
> 
>> diff --git a/tools/testing/selftests/kvm/x86/nested_dirty_log_test.c b/tools/testing/selftests/kvm/x86/nested_dirty_log_test.c
>> index 7e5f9ae1cb00a..b1008724f46dc 100644
>> --- a/tools/testing/selftests/kvm/x86/nested_dirty_log_test.c
>> +++ b/tools/testing/selftests/kvm/x86/nested_dirty_log_test.c
> 
> [ ... ]
> 
>> @@ -192,6 +192,149 @@ static void test_handle_ucall_sync(struct kvm_vm *vm, u64 arg,
>>       }
>>  }
>>
>> +#define PML_LOG_NR_ENTRIES   512
>> +#define PML_HEAD_INDEX               (PML_LOG_NR_ENTRIES - 1)
>> +
>> +#define PML_FULL_MAX_EXITS   64
> 
> [ ... ]
> 
>> +static void l1_pml_vmx_code(struct vmx_pages *vmx)
>> +{
>> +     int nr_pml_full = 0;
>> +
>> +     GUEST_ASSERT(vmx->vmcs_gpa);
>> +     prepare_for_vmx_operation(vmx);
>> +     load_vmcs(vmx);
>> +
>> +     /* PML requires EPT. */
>> +     GUEST_ASSERT(vmx->eptp_gpa);
>> +
>> +     prepare_vmcs(vmx, l2_pml_guest_code);
>> +
>> +     vmwrite(PML_ADDRESS, vmx->pml_gpa);
>> +     vmwrite(GUEST_PML_INDEX, PML_LOG_NR_ENTRIES);
>> +     vmwrite(SECONDARY_VM_EXEC_CONTROL,
>> +             vmread(SECONDARY_VM_EXEC_CONTROL) | SECONDARY_EXEC_ENABLE_PML);
>> +
>> +     vmlaunch();
>> +
>> +     for (;;) {
>> +             u64 reason = vmread(VM_EXIT_REASON);
>> +
>> +             if (reason == EXIT_REASON_VMCALL)
>> +                     break;
>> +
>> +             GUEST_ASSERT_EQ(reason, EXIT_REASON_PML_FULL);
> 
> [Severity: Medium]
> Since the test intentionally sets GUEST_PML_INDEX to 512, shouldn't this
> result in a VM-entry failure instead of a successful entry followed by a
> PML_FULL exit?

It does not, and the test works as intended on both vendors.  KVM does not
check guest_pml_index at VM-entry; it is only consumed in
nested_vmx_write_pml_buffer().

The seed is deliberate: on AMD the APM defines an index outside the valid
range as the trigger for a PML Full exit, and the test uses the same setup
for both vendors to get one deterministic PML_FULL.

Regards,
Nikunj