KVM: arm64: Make table->block changes parallel-aware

stage2_map_walker_try_leaf() and friends now handle stage-2 PTEs
generically, and perform the correct flush when a table PTE is removed.
Additionally, they've been made parallel-aware, using an atomic break
to take ownership of the PTE.

Stop clearing the PTE in the pre-order callback and instead let
stage2_map_walker_try_leaf() deal with it.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221107220006.1895572-1-oliver.upton@linux.dev

arch/arm64/kvm/hyp/pgtable.c

@@ -841,21 +841,12 @@ static int stage2_map_walk_table_pre(const struct kvm_pgtable_visit_ctx *ctx,
 	if (!stage2_leaf_mapping_allowed(ctx, data))
 		return 0;
 
-	kvm_clear_pte(ctx->ptep);
-
-	/*
-	 * Invalidate the whole stage-2, as we may have numerous leaf
-	 * entries below us which would otherwise need invalidating
-	 * individually.
-	 */
-	kvm_call_hyp(__kvm_tlb_flush_vmid, data->mmu);
-
 	ret = stage2_map_walker_try_leaf(ctx, data);
+	if (ret)
+		return ret;
 
-	mm_ops->put_page(ctx->ptep);
 	mm_ops->free_removed_table(childp, ctx->level);
-
-	return ret;
+	return 0;
 }
 
 static int stage2_map_walk_leaf(const struct kvm_pgtable_visit_ctx *ctx,