kvm: mmu: Separate making non-leaf sptes from link_shadow_page

The TDP MMU page fault handler will need to be able to create non-leaf
SPTEs to build up the paging structures. Rather than re-implementing the
function, factor the SPTE creation out of link_shadow_page.

Tested by running kvm-unit-tests and KVM selftests on an Intel Haswell
machine. This series introduced no new failures.

This series can be viewed in Gerrit at:
	https://linux-review.googlesource.com/c/virt/kvm/kvm/+/2538Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20200925212302.3979661-9-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -2573,21 +2573,30 @@ static void shadow_walk_next(struct kvm_shadow_walk_iterator *iterator)
 	__shadow_walk_next(iterator, *iterator->sptep);
 }
 
-static void link_shadow_page(struct kvm_vcpu *vcpu, u64 *sptep,
-			     struct kvm_mmu_page *sp)
+static u64 make_nonleaf_spte(u64 *child_pt, bool ad_disabled)
 {
 	u64 spte;
 
-	BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK);
-
-	spte = __pa(sp->spt) | shadow_present_mask | PT_WRITABLE_MASK |
+	spte = __pa(child_pt) | shadow_present_mask | PT_WRITABLE_MASK |
 	       shadow_user_mask | shadow_x_mask | shadow_me_mask;
 
-	if (sp_ad_disabled(sp))
+	if (ad_disabled)
 		spte |= SPTE_AD_DISABLED_MASK;
 	else
 		spte |= shadow_accessed_mask;
 
+	return spte;
+}
+
+static void link_shadow_page(struct kvm_vcpu *vcpu, u64 *sptep,
+			     struct kvm_mmu_page *sp)
+{
+	u64 spte;
+
+	BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK);
+
+	spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp));
+
 	mmu_spte_set(sptep, spte);
 
 	mmu_page_add_parent_pte(vcpu, sp, sptep);