Re: [PATCH v8 8/8] KVM: SVM: Add Page modification logging support

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

On 9/10/2026 5:52 AM, Sean Christopherson wrote:
> On Tue, Sep 08, 2026, Nikunj A. Dadhania wrote:
>>
>>
>> On 9/7/2026 12:30 PM, sashiko-bot@kernel.org wrote:
>>
>>>> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
>>>> index 73f37b050d0a0..da417c4c59154 100644
>>>> --- a/arch/x86/kvm/svm/nested.c
>>>> +++ b/arch/x86/kvm/svm/nested.c
>>>> @@ -901,6 +901,13 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
>>>>       vmcb02->control.msrpm_base_pa = vmcb01->control.msrpm_base_pa;
>>>>       vmcb_mark_dirty(vmcb02, VMCB_PERM_MAP);
>>>>
>>>> +     /*
>>>> +      * PML is never enabled in hardware for L2.  Make sure that an
>>>> +      * unexpected PML write would trigger a PML_FULL VM-Exit.
>>>> +      */
>>>> +     if (pml)
>>>> +             vmcb02->control.pml_index = -1;
>>>
>>> [Severity: Critical]
>>> By intentionally leaving hardware PML disabled for nested L2 guests in
>>> nested_vmcb02_prepare_control without informing the KVM MMU, doesn't this
>>> cause all memory dirtied by L2 to be silently lost?
>>
>> No, kvm_mmu_page_ad_need_write_protect() returns true when
>> cpu_dirty_log_size is set and the shadow page is in guest_mode,
>> which forces write-protection on L2 SPTEs instead of relying on
>> hardware PML. L2 dirty pages are tracked via write faults, not PML.
>> This is the same approach VMX uses for nested guests.
> 
> And FWIW, because Sashiko's hallucinations made me double check, this series
> doesn't advertise PML to L1, i.e. doesn't need to implement and wire up
> kvm_x86_nested_ops.write_log_dirty() for SVM.

Yes, SVM nested PML is on my radar, I have been working on it along with
selftest coverage in nested_dirty_log_test.  The test covers nVMX as well
-- dirty_log_perf_test -n runs an L2, but never enables PML for it, so
KVM's existing nVMX PML emulation doesn't appear to be exercised today.
Will post it once it's ready for review.

Regards,
Nikunj