Re: [PATCH v6 7/7] KVM: SVM: Add Page modification logging support

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

On 4/22/2026 4:45 AM, Sean Christopherson wrote:
> On Tue, Apr 21, 2026, Yosry Ahmed wrote:
>>> @@ -3253,6 +3278,54 @@ static int bus_lock_exit(struct kvm_vcpu *vcpu)
>>>  	return 0;
>>>  }
>>>  
>>> +void svm_update_cpu_dirty_logging(struct kvm_vcpu *vcpu)
>>> +{
>>> +	struct vcpu_svm *svm = to_svm(vcpu);
>>> +
>>> +	if (WARN_ON_ONCE(!vcpu->kvm->arch.cpu_dirty_log_size))
>>> +		return;
>>> +
>>> +	guard(svm_vmcb01)(vcpu);
>>
>> Why do we need this? Why not update vmcb01 directly below?

Are you suggesting something like below:

void svm_update_cpu_dirty_logging(struct kvm_vcpu *vcpu)
{
	struct vcpu_svm *svm = to_svm(vcpu);
	struct vmcb *vmcb01 = svm->vmcb01.ptr;

	if (WARN_ON_ONCE(!vcpu->kvm->arch.cpu_dirty_log_size))
		return;

	/*
	 * Note, nr_memslots_dirty_logging can be changed concurrent with this
	 * code, but in that case another update request will be made and so
	 * the guest will never run with a stale PML value.
	 */
	if (atomic_read(&vcpu->kvm->nr_memslots_dirty_logging))
		vmcb01->control.nested_ctl |= SVM_NESTED_CTL_PML_ENABLE;
	else
		vmcb01->control.nested_ctl &= ~SVM_NESTED_CTL_PML_ENABLE;

	vmcb_mark_dirty(vmcb01, VMCB_NPT);
}

> 
> Yeah, I don't see the point of patch 6.  guard(svm_vmcb01) doesn't exist in the
> current code base because it's not needed.

Sure, I can drop patch6. I wanted to keep the code similar.

> 
>> IIUC VMX needs this because it updates the loaded VMCS, but the VMCB is
>> just memory.
> 
> Yep.  If VMWRITE could target an arbitrary VMCS pointer, VMX wouldn't need the
> helper either.

Regards
Nikunj