KVM: x86/mmu: Expand on the comment in kvm_vcpu_ad_need_write_protect()

Expand the comment about need to use write-protection for nested EPT
when PML is enabled to clarify that the tagging is a nop when PML is
_not_ enabled.  Without the clarification, omitting the PML check looks
wrong at first^Wfifth glance.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210213005015.1651772-8-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu_internal.h

@@ -84,7 +84,10 @@ static inline bool kvm_vcpu_ad_need_write_protect(struct kvm_vcpu *vcpu)
 	 * When using the EPT page-modification log, the GPAs in the log
 	 * would come from L2 rather than L1.  Therefore, we need to rely
 	 * on write protection to record dirty pages.  This also bypasses
-	 * PML, since writes now result in a vmexit.
+	 * PML, since writes now result in a vmexit.  Note, this helper will
+	 * tag SPTEs as needing write-protection even if PML is disabled or
+	 * unsupported, but that's ok because the tag is consumed if and only
+	 * if PML is enabled.  Omit the PML check to save a few uops.
 	 */
 	return vcpu->arch.mmu == &vcpu->arch.guest_mmu;
 }