KVM: arm64: Split init and set for table PTE

Create a helper to initialize a table and directly call
smp_store_release() to install it (for now). Prepare for a subsequent
change that generalizes PTE writes with a helper.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221107215644.1895162-11-oliver.upton@linux.dev

arch/arm64/kvm/hyp/pgtable.c

@@ -136,16 +136,13 @@ static void kvm_clear_pte(kvm_pte_t *ptep)
 	WRITE_ONCE(*ptep, 0);
 }
 
-static void kvm_set_table_pte(kvm_pte_t *ptep, kvm_pte_t *childp,
-			      struct kvm_pgtable_mm_ops *mm_ops)
+static kvm_pte_t kvm_init_table_pte(kvm_pte_t *childp, struct kvm_pgtable_mm_ops *mm_ops)
 {
-	kvm_pte_t old = *ptep, pte = kvm_phys_to_pte(mm_ops->virt_to_phys(childp));
+	kvm_pte_t pte = kvm_phys_to_pte(mm_ops->virt_to_phys(childp));
 
 	pte |= FIELD_PREP(KVM_PTE_TYPE, KVM_PTE_TYPE_TABLE);
 	pte |= KVM_PTE_VALID;
-
-	WARN_ON(kvm_pte_valid(old));
-	smp_store_release(ptep, pte);
+	return pte;
 }
 
 static kvm_pte_t kvm_init_valid_leaf_pte(u64 pa, kvm_pte_t attr, u32 level)
@@ -413,7 +410,7 @@ static bool hyp_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx,
 static int hyp_map_walker(const struct kvm_pgtable_visit_ctx *ctx,
 			  enum kvm_pgtable_walk_flags visit)
 {
-	kvm_pte_t *childp;
+	kvm_pte_t *childp, new;
 	struct hyp_map_data *data = ctx->arg;
 	struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
 
@@ -427,8 +424,10 @@ static int hyp_map_walker(const struct kvm_pgtable_visit_ctx *ctx,
 	if (!childp)
 		return -ENOMEM;
 
-	kvm_set_table_pte(ctx->ptep, childp, mm_ops);
+	new = kvm_init_table_pte(childp, mm_ops);
 	mm_ops->get_page(ctx->ptep);
+	smp_store_release(ctx->ptep, new);
+
 	return 0;
 }
 
@@ -796,7 +795,7 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,
 				struct stage2_map_data *data)
 {
 	struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
-	kvm_pte_t *childp;
+	kvm_pte_t *childp, new;
 	int ret;
 
 	ret = stage2_map_walker_try_leaf(ctx, data);
@@ -821,8 +820,9 @@ static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,
 	if (stage2_pte_is_counted(ctx->old))
 		stage2_put_pte(ctx, data->mmu, mm_ops);
 
-	kvm_set_table_pte(ctx->ptep, childp, mm_ops);
+	new = kvm_init_table_pte(childp, mm_ops);
 	mm_ops->get_page(ctx->ptep);
+	smp_store_release(ctx->ptep, new);
 
 	return 0;
 }