24 files changed, 1037 insertions, 33 deletions

diff --git a/src/core/CMakeLists.txt b/src/core/CMakeLists.txt
index 26f727d96..23fd6e920 100644
--- a/src/core/CMakeLists.txt
+++ b/src/core/CMakeLists.txt
@@ -32,6 +32,8 @@ add_library(core STATIC
     file_sys/control_metadata.h
     file_sys/directory.h
     file_sys/errors.h
+    file_sys/fsmitm_romfsbuild.cpp
+    file_sys/fsmitm_romfsbuild.h
     file_sys/mode.h
     file_sys/nca_metadata.cpp
     file_sys/nca_metadata.h
@@ -59,10 +61,13 @@ add_library(core STATIC
     file_sys/vfs.h
     file_sys/vfs_concat.cpp
     file_sys/vfs_concat.h
+    file_sys/vfs_layered.cpp
+    file_sys/vfs_layered.h
     file_sys/vfs_offset.cpp
     file_sys/vfs_offset.h
     file_sys/vfs_real.cpp
     file_sys/vfs_real.h
+    file_sys/vfs_static.h
     file_sys/vfs_vector.cpp
     file_sys/vfs_vector.h
     file_sys/xts_archive.cpp
diff --git a/src/core/file_sys/bis_factory.cpp b/src/core/file_sys/bis_factory.cpp
index 205492897..6102ef476 100644
--- a/src/core/file_sys/bis_factory.cpp
+++ b/src/core/file_sys/bis_factory.cpp
@@ -2,13 +2,14 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
+#include <fmt/format.h>
 #include "core/file_sys/bis_factory.h"
 #include "core/file_sys/registered_cache.h"
 
 namespace FileSys {
 
-BISFactory::BISFactory(VirtualDir nand_root_)
-    : nand_root(std::move(nand_root_)),
+BISFactory::BISFactory(VirtualDir nand_root_, VirtualDir load_root_)
+    : nand_root(std::move(nand_root_)), load_root(std::move(load_root_)),
       sysnand_cache(std::make_shared<RegisteredCache>(
           GetOrCreateDirectoryRelative(nand_root, "/system/Contents/registered"))),
       usrnand_cache(std::make_shared<RegisteredCache>(
@@ -24,4 +25,11 @@ std::shared_ptr<RegisteredCache> BISFactory::GetUserNANDContents() const {
     return usrnand_cache;
 }
 
+VirtualDir BISFactory::GetModificationLoadRoot(u64 title_id) const {
+    // LayeredFS doesn't work on updates and title id-less homebrew
+    if (title_id == 0 || (title_id & 0x800) > 0)
+        return nullptr;
+    return GetOrCreateDirectoryRelative(load_root, fmt::format("/{:016X}", title_id));
+}
+
 } // namespace FileSys
diff --git a/src/core/file_sys/bis_factory.h b/src/core/file_sys/bis_factory.h
index 9523dd864..c352e0925 100644
--- a/src/core/file_sys/bis_factory.h
+++ b/src/core/file_sys/bis_factory.h
@@ -17,14 +17,17 @@ class RegisteredCache;
 /// registered caches.
 class BISFactory {
 public:
-    explicit BISFactory(VirtualDir nand_root);
+    explicit BISFactory(VirtualDir nand_root, VirtualDir load_root);
     ~BISFactory();
 
     std::shared_ptr<RegisteredCache> GetSystemNANDContents() const;
     std::shared_ptr<RegisteredCache> GetUserNANDContents() const;
 
+    VirtualDir GetModificationLoadRoot(u64 title_id) const;
+
 private:
     VirtualDir nand_root;
+    VirtualDir load_root;
 
     std::shared_ptr<RegisteredCache> sysnand_cache;
     std::shared_ptr<RegisteredCache> usrnand_cache;
diff --git a/src/core/file_sys/fsmitm_romfsbuild.cpp b/src/core/file_sys/fsmitm_romfsbuild.cpp
new file mode 100644
index 000000000..21fc3d796
--- /dev/null
+++ b/src/core/file_sys/fsmitm_romfsbuild.cpp
@@ -0,0 +1,372 @@
+/*
+ * Copyright (c) 2018 Atmosphère-NX
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * Adapted by DarkLordZach for use/interaction with yuzu
+ *
+ * Modifications Copyright 2018 yuzu emulator team
+ * Licensed under GPLv2 or any later version
+ * Refer to the license.txt file included.
+ */
+
+#include <cstring>
+#include "common/assert.h"
+#include "core/file_sys/fsmitm_romfsbuild.h"
+#include "core/file_sys/vfs.h"
+#include "core/file_sys/vfs_vector.h"
+
+namespace FileSys {
+
+constexpr u64 FS_MAX_PATH = 0x301;
+
+constexpr u32 ROMFS_ENTRY_EMPTY = 0xFFFFFFFF;
+constexpr u32 ROMFS_FILEPARTITION_OFS = 0x200;
+
+// Types for building a RomFS.
+struct RomFSHeader {
+    u64 header_size;
+    u64 dir_hash_table_ofs;
+    u64 dir_hash_table_size;
+    u64 dir_table_ofs;
+    u64 dir_table_size;
+    u64 file_hash_table_ofs;
+    u64 file_hash_table_size;
+    u64 file_table_ofs;
+    u64 file_table_size;
+    u64 file_partition_ofs;
+};
+static_assert(sizeof(RomFSHeader) == 0x50, "RomFSHeader has incorrect size.");
+
+struct RomFSDirectoryEntry {
+    u32 parent;
+    u32 sibling;
+    u32 child;
+    u32 file;
+    u32 hash;
+    u32 name_size;
+};
+static_assert(sizeof(RomFSDirectoryEntry) == 0x18, "RomFSDirectoryEntry has incorrect size.");
+
+struct RomFSFileEntry {
+    u32 parent;
+    u32 sibling;
+    u64 offset;
+    u64 size;
+    u32 hash;
+    u32 name_size;
+};
+static_assert(sizeof(RomFSFileEntry) == 0x20, "RomFSFileEntry has incorrect size.");
+
+struct RomFSBuildFileContext;
+
+struct RomFSBuildDirectoryContext {
+    std::string path = "";
+    u32 cur_path_ofs = 0;
+    u32 path_len = 0;
+    u32 entry_offset = 0;
+    std::shared_ptr<RomFSBuildDirectoryContext> parent;
+    std::shared_ptr<RomFSBuildDirectoryContext> child;
+    std::shared_ptr<RomFSBuildDirectoryContext> sibling;
+    std::shared_ptr<RomFSBuildFileContext> file;
+};
+
+struct RomFSBuildFileContext {
+    std::string path = "";
+    u32 cur_path_ofs = 0;
+    u32 path_len = 0;
+    u32 entry_offset = 0;
+    u64 offset = 0;
+    u64 size = 0;
+    std::shared_ptr<RomFSBuildDirectoryContext> parent;
+    std::shared_ptr<RomFSBuildFileContext> sibling;
+    VirtualFile source = nullptr;
+
+    RomFSBuildFileContext() : path(""), cur_path_ofs(0), path_len(0) {}
+};
+
+static u32 romfs_calc_path_hash(u32 parent, std::string path, u32 start, size_t path_len) {
+    u32 hash = parent ^ 123456789;
+    for (u32 i = 0; i < path_len; i++) {
+        hash = (hash >> 5) | (hash << 27);
+        hash ^= path[start + i];
+    }
+
+    return hash;
+}
+
+static u32 romfs_get_hash_table_count(u32 num_entries) {
+    if (num_entries < 3) {
+        return 3;
+    } else if (num_entries < 19) {
+        return num_entries | 1;
+    }
+    u32 count = num_entries;
+    while (count % 2 == 0 || count % 3 == 0 || count % 5 == 0 || count % 7 == 0 ||
+           count % 11 == 0 || count % 13 == 0 || count % 17 == 0) {
+        count++;
+    }
+    return count;
+}
+
+void RomFSBuildContext::VisitDirectory(VirtualDir root_romfs,
+                                       std::shared_ptr<RomFSBuildDirectoryContext> parent) {
+    std::vector<std::shared_ptr<RomFSBuildDirectoryContext>> child_dirs;
+
+    VirtualDir dir;
+
+    if (parent->path_len == 0)
+        dir = root_romfs;
+    else
+        dir = root_romfs->GetDirectoryRelative(parent->path);
+
+    const auto entries = dir->GetEntries();
+
+    for (const auto& kv : entries) {
+        if (kv.second == VfsEntryType::Directory) {
+            const auto child = std::make_shared<RomFSBuildDirectoryContext>();
+            // Set child's path.
+            child->cur_path_ofs = parent->path_len + 1;
+            child->path_len = child->cur_path_ofs + kv.first.size();
+            child->path = parent->path + "/" + kv.first;
+
+            // Sanity check on path_len
+            ASSERT(child->path_len < FS_MAX_PATH);
+
+            if (AddDirectory(parent, child)) {
+                child_dirs.push_back(child);
+            }
+        } else {
+            const auto child = std::make_shared<RomFSBuildFileContext>();
+            // Set child's path.
+            child->cur_path_ofs = parent->path_len + 1;
+            child->path_len = child->cur_path_ofs + kv.first.size();
+            child->path = parent->path + "/" + kv.first;
+
+            // Sanity check on path_len
+            ASSERT(child->path_len < FS_MAX_PATH);
+
+            child->source = root_romfs->GetFileRelative(child->path);
+
+            child->size = child->source->GetSize();
+
+            AddFile(parent, child);
+        }
+    }
+
+    for (auto& child : child_dirs) {
+        this->VisitDirectory(root_romfs, child);
+    }
+}
+
+bool RomFSBuildContext::AddDirectory(std::shared_ptr<RomFSBuildDirectoryContext> parent_dir_ctx,
+                                     std::shared_ptr<RomFSBuildDirectoryContext> dir_ctx) {
+    // Check whether it's already in the known directories.
+    const auto existing = directories.find(dir_ctx->path);
+    if (existing != directories.end())
+        return false;
+
+    // Add a new directory.
+    num_dirs++;
+    dir_table_size +=
+        sizeof(RomFSDirectoryEntry) + ((dir_ctx->path_len - dir_ctx->cur_path_ofs + 3) & ~3);
+    dir_ctx->parent = parent_dir_ctx;
+    directories.emplace(dir_ctx->path, dir_ctx);
+
+    return true;
+}
+
+bool RomFSBuildContext::AddFile(std::shared_ptr<RomFSBuildDirectoryContext> parent_dir_ctx,
+                                std::shared_ptr<RomFSBuildFileContext> file_ctx) {
+    // Check whether it's already in the known files.
+    const auto existing = files.find(file_ctx->path);
+    if (existing != files.end()) {
+        return false;
+    }
+
+    // Add a new file.
+    num_files++;
+    file_table_size +=
+        sizeof(RomFSFileEntry) + ((file_ctx->path_len - file_ctx->cur_path_ofs + 3) & ~3);
+    file_ctx->parent = parent_dir_ctx;
+    files.emplace(file_ctx->path, file_ctx);
+
+    return true;
+}
+
+RomFSBuildContext::RomFSBuildContext(VirtualDir base_) : base(std::move(base_)) {
+    root = std::make_shared<RomFSBuildDirectoryContext>();
+    root->path = "\0";
+    directories.emplace(root->path, root);
+    num_dirs = 1;
+    dir_table_size = 0x18;
+
+    VisitDirectory(base, root);
+}
+
+RomFSBuildContext::~RomFSBuildContext() = default;
+
+std::map<u64, VirtualFile> RomFSBuildContext::Build() {
+    const auto dir_hash_table_entry_count = romfs_get_hash_table_count(num_dirs);
+    const auto file_hash_table_entry_count = romfs_get_hash_table_count(num_files);
+    dir_hash_table_size = 4 * dir_hash_table_entry_count;
+    file_hash_table_size = 4 * file_hash_table_entry_count;
+
+    // Assign metadata pointers
+    RomFSHeader header{};
+
+    std::vector<u32> dir_hash_table(dir_hash_table_entry_count, ROMFS_ENTRY_EMPTY);
+    std::vector<u32> file_hash_table(file_hash_table_entry_count, ROMFS_ENTRY_EMPTY);
+
+    std::vector<u8> dir_table(dir_table_size);
+    std::vector<u8> file_table(file_table_size);
+
+    // Clear out hash tables.
+    for (u32 i = 0; i < dir_hash_table_entry_count; i++)
+        dir_hash_table[i] = ROMFS_ENTRY_EMPTY;
+    for (u32 i = 0; i < file_hash_table_entry_count; i++)
+        file_hash_table[i] = ROMFS_ENTRY_EMPTY;
+
+    std::shared_ptr<RomFSBuildFileContext> cur_file;
+
+    // Determine file offsets.
+    u32 entry_offset = 0;
+    std::shared_ptr<RomFSBuildFileContext> prev_file = nullptr;
+    for (const auto& it : files) {
+        cur_file = it.second;
+        file_partition_size = (file_partition_size + 0xFULL) & ~0xFULL;
+        cur_file->offset = file_partition_size;
+        file_partition_size += cur_file->size;
+        cur_file->entry_offset = entry_offset;
+        entry_offset +=
+            sizeof(RomFSFileEntry) + ((cur_file->path_len - cur_file->cur_path_ofs + 3) & ~3);
+        prev_file = cur_file;
+    }
+    // Assign deferred parent/sibling ownership.
+    for (auto it = files.rbegin(); it != files.rend(); ++it) {
+        cur_file = it->second;
+        cur_file->sibling = cur_file->parent->file;
+        cur_file->parent->file = cur_file;
+    }
+
+    std::shared_ptr<RomFSBuildDirectoryContext> cur_dir;
+
+    // Determine directory offsets.
+    entry_offset = 0;
+    for (const auto& it : directories) {
+        cur_dir = it.second;
+        cur_dir->entry_offset = entry_offset;
+        entry_offset +=
+            sizeof(RomFSDirectoryEntry) + ((cur_dir->path_len - cur_dir->cur_path_ofs + 3) & ~3);
+    }
+    // Assign deferred parent/sibling ownership.
+    for (auto it = directories.rbegin(); it->second != root; ++it) {
+        cur_dir = it->second;
+        cur_dir->sibling = cur_dir->parent->child;
+        cur_dir->parent->child = cur_dir;
+    }
+
+    std::map<u64, VirtualFile> out;
+
+    // Populate file tables.
+    for (const auto& it : files) {
+        cur_file = it.second;
+        RomFSFileEntry cur_entry{};
+
+        cur_entry.parent = cur_file->parent->entry_offset;
+        cur_entry.sibling =
+            cur_file->sibling == nullptr ? ROMFS_ENTRY_EMPTY : cur_file->sibling->entry_offset;
+        cur_entry.offset = cur_file->offset;
+        cur_entry.size = cur_file->size;
+
+        const auto name_size = cur_file->path_len - cur_file->cur_path_ofs;
+        const auto hash = romfs_calc_path_hash(cur_file->parent->entry_offset, cur_file->path,
+                                               cur_file->cur_path_ofs, name_size);
+        cur_entry.hash = file_hash_table[hash % file_hash_table_entry_count];
+        file_hash_table[hash % file_hash_table_entry_count] = cur_file->entry_offset;
+
+        cur_entry.name_size = name_size;
+
+        out.emplace(cur_file->offset + ROMFS_FILEPARTITION_OFS, cur_file->source);
+        std::memcpy(file_table.data() + cur_file->entry_offset, &cur_entry, sizeof(RomFSFileEntry));
+        std::memset(file_table.data() + cur_file->entry_offset + sizeof(RomFSFileEntry), 0,
+                    (cur_entry.name_size + 3) & ~3);
+        std::memcpy(file_table.data() + cur_file->entry_offset + sizeof(RomFSFileEntry),
+                    cur_file->path.data() + cur_file->cur_path_ofs, name_size);
+    }
+
+    // Populate dir tables.
+    for (const auto& it : directories) {
+        cur_dir = it.second;
+        RomFSDirectoryEntry cur_entry{};
+
+        cur_entry.parent = cur_dir == root ? 0 : cur_dir->parent->entry_offset;
+        cur_entry.sibling =
+            cur_dir->sibling == nullptr ? ROMFS_ENTRY_EMPTY : cur_dir->sibling->entry_offset;
+        cur_entry.child =
+            cur_dir->child == nullptr ? ROMFS_ENTRY_EMPTY : cur_dir->child->entry_offset;
+        cur_entry.file = cur_dir->file == nullptr ? ROMFS_ENTRY_EMPTY : cur_dir->file->entry_offset;
+
+        const auto name_size = cur_dir->path_len - cur_dir->cur_path_ofs;
+        const auto hash = romfs_calc_path_hash(cur_dir == root ? 0 : cur_dir->parent->entry_offset,
+                                               cur_dir->path, cur_dir->cur_path_ofs, name_size);
+        cur_entry.hash = dir_hash_table[hash % dir_hash_table_entry_count];
+        dir_hash_table[hash % dir_hash_table_entry_count] = cur_dir->entry_offset;
+
+        cur_entry.name_size = name_size;
+
+        std::memcpy(dir_table.data() + cur_dir->entry_offset, &cur_entry,
+                    sizeof(RomFSDirectoryEntry));
+        std::memcpy(dir_table.data() + cur_dir->entry_offset, &cur_entry,
+                    sizeof(RomFSDirectoryEntry));
+        std::memset(dir_table.data() + cur_dir->entry_offset + sizeof(RomFSDirectoryEntry), 0,
+                    (cur_entry.name_size + 3) & ~3);
+        std::memcpy(dir_table.data() + cur_dir->entry_offset + sizeof(RomFSDirectoryEntry),
+                    cur_dir->path.data() + cur_dir->cur_path_ofs, name_size);
+    }
+
+    // Set header fields.
+    header.header_size = sizeof(RomFSHeader);
+    header.file_hash_table_size = file_hash_table_size;
+    header.file_table_size = file_table_size;
+    header.dir_hash_table_size = dir_hash_table_size;
+    header.dir_table_size = dir_table_size;
+    header.file_partition_ofs = ROMFS_FILEPARTITION_OFS;
+    header.dir_hash_table_ofs = (header.file_partition_ofs + file_partition_size + 3ULL) & ~3ULL;
+    header.dir_table_ofs = header.dir_hash_table_ofs + header.dir_hash_table_size;
+    header.file_hash_table_ofs = header.dir_table_ofs + header.dir_table_size;
+    header.file_table_ofs = header.file_hash_table_ofs + header.file_hash_table_size;
+
+    std::vector<u8> header_data(sizeof(RomFSHeader));
+    std::memcpy(header_data.data(), &header, header_data.size());
+    out.emplace(0, std::make_shared<VectorVfsFile>(header_data));
+
+    std::vector<u8> metadata(file_hash_table_size + file_table_size + dir_hash_table_size +
+                             dir_table_size);
+    auto index = 0;
+    std::memcpy(metadata.data(), dir_hash_table.data(), dir_hash_table.size() * sizeof(u32));
+    index += dir_hash_table.size() * sizeof(u32);
+    std::memcpy(metadata.data() + index, dir_table.data(), dir_table.size());
+    index += dir_table.size();
+    std::memcpy(metadata.data() + index, file_hash_table.data(),
+                file_hash_table.size() * sizeof(u32));
+    index += file_hash_table.size() * sizeof(u32);
+    std::memcpy(metadata.data() + index, file_table.data(), file_table.size());
+    out.emplace(header.dir_hash_table_ofs, std::make_shared<VectorVfsFile>(metadata));
+
+    return out;
+}
+
+} // namespace FileSys
diff --git a/src/core/file_sys/fsmitm_romfsbuild.h b/src/core/file_sys/fsmitm_romfsbuild.h
new file mode 100644
index 000000000..b0c3c123b
--- /dev/null
+++ b/src/core/file_sys/fsmitm_romfsbuild.h
@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2018 Atmosphère-NX
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * Adapted by DarkLordZach for use/interaction with yuzu
+ *
+ * Modifications Copyright 2018 yuzu emulator team
+ * Licensed under GPLv2 or any later version
+ * Refer to the license.txt file included.
+ */
+
+#pragma once
+
+#include <map>
+#include <memory>
+#include <string>
+#include <boost/detail/container_fwd.hpp>
+#include "common/common_types.h"
+#include "core/file_sys/vfs.h"
+
+namespace FileSys {
+
+struct RomFSBuildDirectoryContext;
+struct RomFSBuildFileContext;
+struct RomFSDirectoryEntry;
+struct RomFSFileEntry;
+
+class RomFSBuildContext {
+public:
+    explicit RomFSBuildContext(VirtualDir base);
+    ~RomFSBuildContext();
+
+    // This finalizes the context.
+    std::map<u64, VirtualFile> Build();
+
+private:
+    VirtualDir base;
+    std::shared_ptr<RomFSBuildDirectoryContext> root;
+    std::map<std::string, std::shared_ptr<RomFSBuildDirectoryContext>, std::less<>> directories;
+    std::map<std::string, std::shared_ptr<RomFSBuildFileContext>, std::less<>> files;
+    u64 num_dirs = 0;
+    u64 num_files = 0;
+    u64 dir_table_size = 0;
+    u64 file_table_size = 0;
+    u64 dir_hash_table_size = 0;
+    u64 file_hash_table_size = 0;
+    u64 file_partition_size = 0;
+
+    void VisitDirectory(VirtualDir filesys, std::shared_ptr<RomFSBuildDirectoryContext> parent);
+
+    bool AddDirectory(std::shared_ptr<RomFSBuildDirectoryContext> parent_dir_ctx,
+                      std::shared_ptr<RomFSBuildDirectoryContext> dir_ctx);
+    bool AddFile(std::shared_ptr<RomFSBuildDirectoryContext> parent_dir_ctx,
+                 std::shared_ptr<RomFSBuildFileContext> file_ctx);
+};
+
+} // namespace FileSys
diff --git a/src/core/file_sys/patch_manager.cpp b/src/core/file_sys/patch_manager.cpp
index aebc69d52..af3f9a78f 100644
--- a/src/core/file_sys/patch_manager.cpp
+++ b/src/core/file_sys/patch_manager.cpp
@@ -11,6 +11,7 @@
 #include "core/file_sys/patch_manager.h"
 #include "core/file_sys/registered_cache.h"
 #include "core/file_sys/romfs.h"
+#include "core/file_sys/vfs_layered.h"
 #include "core/hle/service/filesystem/filesystem.h"
 #include "core/loader/loader.h"
 
@@ -31,8 +32,9 @@ std::string FormatTitleVersion(u32 version, TitleVersionFormat format) {
     return fmt::format("v{}.{}.{}", bytes[3], bytes[2], bytes[1]);
 }
 
-constexpr std::array<const char*, 1> PATCH_TYPE_NAMES{
+constexpr std::array<const char*, 2> PATCH_TYPE_NAMES{
     "Update",
+    "LayeredFS",
 };
 
 std::string FormatPatchTypeName(PatchType type) {
@@ -66,6 +68,42 @@ VirtualDir PatchManager::PatchExeFS(VirtualDir exefs) const {
     return exefs;
 }
 
+static void ApplyLayeredFS(VirtualFile& romfs, u64 title_id, ContentRecordType type) {
+    const auto load_dir = Service::FileSystem::GetModificationLoadRoot(title_id);
+    if (type == ContentRecordType::Program && load_dir != nullptr && load_dir->GetSize() > 0) {
+        auto extracted = ExtractRomFS(romfs);
+
+        if (extracted != nullptr) {
+            auto patch_dirs = load_dir->GetSubdirectories();
+            std::sort(patch_dirs.begin(), patch_dirs.end(),
+                      [](const VirtualDir& l, const VirtualDir& r) {
+                          return l->GetName() < r->GetName();
+                      });
+
+            std::vector<VirtualDir> layers;
+            layers.reserve(patch_dirs.size() + 1);
+            for (const auto& subdir : patch_dirs) {
+                auto romfs_dir = subdir->GetSubdirectory("romfs");
+                if (romfs_dir != nullptr)
+                    layers.push_back(std::move(romfs_dir));
+            }
+
+            layers.push_back(std::move(extracted));
+
+            const auto layered = LayerDirectories(layers);
+
+            if (layered != nullptr) {
+                auto packed = CreateRomFS(layered);
+
+                if (packed != nullptr) {
+                    LOG_INFO(Loader, "    RomFS: LayeredFS patches applied successfully");
+                    romfs = std::move(packed);
+                }
+            }
+        }
+    }
+}
+
 VirtualFile PatchManager::PatchRomFS(VirtualFile romfs, u64 ivfc_offset,
                                      ContentRecordType type) const {
     LOG_INFO(Loader, "Patching RomFS for title_id={:016X}, type={:02X}", title_id,
@@ -89,6 +127,9 @@ VirtualFile PatchManager::PatchRomFS(VirtualFile romfs, u64 ivfc_offset,
         }
     }
 
+    // LayeredFS
+    ApplyLayeredFS(romfs, title_id, type);
+
     return romfs;
 }
 
@@ -114,6 +155,10 @@ std::map<PatchType, std::string> PatchManager::GetPatchVersionNames() const {
         }
     }
 
+    const auto lfs_dir = Service::FileSystem::GetModificationLoadRoot(title_id);
+    if (lfs_dir != nullptr && lfs_dir->GetSize() > 0)
+        out.insert_or_assign(PatchType::LayeredFS, "");
+
     return out;
 }
 
diff --git a/src/core/file_sys/patch_manager.h b/src/core/file_sys/patch_manager.h
index 209cab1dc..464f17515 100644
--- a/src/core/file_sys/patch_manager.h
+++ b/src/core/file_sys/patch_manager.h
@@ -26,6 +26,7 @@ std::string FormatTitleVersion(u32 version,
 
 enum class PatchType {
     Update,
+    LayeredFS,
 };
 
 std::string FormatPatchTypeName(PatchType type);
@@ -42,6 +43,7 @@ public:
 
     // Currently tracked RomFS patches:
     // - Game Updates
+    // - LayeredFS
     VirtualFile PatchRomFS(VirtualFile base, u64 ivfc_offset,
                            ContentRecordType type = ContentRecordType::Program) const;
 
diff --git a/src/core/file_sys/registered_cache.cpp b/src/core/file_sys/registered_cache.cpp
index dad7ae10b..653ef2e7b 100644
--- a/src/core/file_sys/registered_cache.cpp
+++ b/src/core/file_sys/registered_cache.cpp
@@ -18,6 +18,10 @@
 #include "core/loader/loader.h"
 
 namespace FileSys {
+
+// The size of blocks to use when vfs raw copying into nand.
+constexpr size_t VFS_RC_LARGE_COPY_BLOCK = 0x400000;
+
 std::string RegisteredCacheEntry::DebugInfo() const {
     return fmt::format("title_id={:016X}, content_type={:02X}", title_id, static_cast<u8>(type));
 }
@@ -121,7 +125,7 @@ VirtualFile RegisteredCache::OpenFileOrDirectoryConcat(const VirtualDir& dir,
             if (concat.empty())
                 return nullptr;
 
-            file = FileSys::ConcatenateFiles(concat);
+            file = FileSys::ConcatenateFiles(concat, concat.front()->GetName());
         }
 
         return file;
@@ -480,7 +484,8 @@ InstallResult RegisteredCache::RawInstallNCA(std::shared_ptr<NCA> nca, const Vfs
     auto out = dir->CreateFileRelative(path);
     if (out == nullptr)
         return InstallResult::ErrorCopyFailed;
-    return copy(in, out) ? InstallResult::Success : InstallResult::ErrorCopyFailed;
+    return copy(in, out, VFS_RC_LARGE_COPY_BLOCK) ? InstallResult::Success
+                                                  : InstallResult::ErrorCopyFailed;
 }
 
 bool RegisteredCache::RawInstallYuzuMeta(const CNMT& cnmt) {
diff --git a/src/core/file_sys/registered_cache.h b/src/core/file_sys/registered_cache.h
index f487b0cf0..c0cd59fc5 100644
--- a/src/core/file_sys/registered_cache.h
+++ b/src/core/file_sys/registered_cache.h
@@ -27,7 +27,7 @@ struct ContentRecord;
 
 using NcaID = std::array<u8, 0x10>;
 using RegisteredCacheParsingFunction = std::function<VirtualFile(const VirtualFile&, const NcaID&)>;
-using VfsCopyFunction = std::function<bool(VirtualFile, VirtualFile)>;
+using VfsCopyFunction = std::function<bool(const VirtualFile&, const VirtualFile&, size_t)>;
 
 enum class InstallResult {
     Success,
diff --git a/src/core/file_sys/romfs.cpp b/src/core/file_sys/romfs.cpp
index 9f6e41cdf..205284a4d 100644
--- a/src/core/file_sys/romfs.cpp
+++ b/src/core/file_sys/romfs.cpp
@@ -4,8 +4,10 @@
 
 #include "common/common_types.h"
 #include "common/swap.h"
+#include "core/file_sys/fsmitm_romfsbuild.h"
 #include "core/file_sys/romfs.h"
 #include "core/file_sys/vfs.h"
+#include "core/file_sys/vfs_concat.h"
 #include "core/file_sys/vfs_offset.h"
 #include "core/file_sys/vfs_vector.h"
 
@@ -98,7 +100,7 @@ void ProcessDirectory(VirtualFile file, std::size_t dir_offset, std::size_t file
     }
 }
 
-VirtualDir ExtractRomFS(VirtualFile file) {
+VirtualDir ExtractRomFS(VirtualFile file, RomFSExtractionType type) {
     RomFSHeader header{};
     if (file->ReadObject(&header) != sizeof(RomFSHeader))
         return nullptr;
@@ -117,9 +119,22 @@ VirtualDir ExtractRomFS(VirtualFile file) {
 
     VirtualDir out = std::move(root);
 
-    while (out->GetSubdirectory("") != nullptr)
-        out = out->GetSubdirectory("");
+    while (out->GetSubdirectories().size() == 1 && out->GetFiles().empty()) {
+        if (out->GetSubdirectories().front()->GetName() == "data" &&
+            type == RomFSExtractionType::Truncated)
+            break;
+        out = out->GetSubdirectories().front();
+    }
 
     return out;
 }
+
+VirtualFile CreateRomFS(VirtualDir dir) {
+    if (dir == nullptr)
+        return nullptr;
+
+    RomFSBuildContext ctx{dir};
+    return ConcatenateFiles<0>(ctx.Build(), dir->GetName());
+}
+
 } // namespace FileSys
diff --git a/src/core/file_sys/romfs.h b/src/core/file_sys/romfs.h
index e54a7d7a9..ecd1eb725 100644
--- a/src/core/file_sys/romfs.h
+++ b/src/core/file_sys/romfs.h
@@ -5,6 +5,7 @@
 #pragma once
 
 #include <array>
+#include <map>
 #include "common/common_funcs.h"
 #include "common/common_types.h"
 #include "common/swap.h"
@@ -12,6 +13,8 @@
 
 namespace FileSys {
 
+struct RomFSHeader;
+
 struct IVFCLevel {
     u64_le offset;
     u64_le size;
@@ -29,8 +32,18 @@ struct IVFCHeader {
 };
 static_assert(sizeof(IVFCHeader) == 0xE0, "IVFCHeader has incorrect size.");
 
+enum class RomFSExtractionType {
+    Full,      // Includes data directory
+    Truncated, // Traverses into data directory
+};
+
 // Converts a RomFS binary blob to VFS Filesystem
 // Returns nullptr on failure
-VirtualDir ExtractRomFS(VirtualFile file);
+VirtualDir ExtractRomFS(VirtualFile file,
+                        RomFSExtractionType type = RomFSExtractionType::Truncated);
+
+// Converts a VFS filesystem into a RomFS binary
+// Returns nullptr on failure
+VirtualFile CreateRomFS(VirtualDir dir);
 
 } // namespace FileSys
diff --git a/src/core/file_sys/vfs.cpp b/src/core/file_sys/vfs.cpp
index d7b52abfd..5fbea1739 100644
--- a/src/core/file_sys/vfs.cpp
+++ b/src/core/file_sys/vfs.cpp
@@ -399,6 +399,15 @@ bool VfsDirectory::Copy(std::string_view src, std::string_view dest) {
     return f2->WriteBytes(f1->ReadAllBytes()) == f1->GetSize();
 }
 
+std::map<std::string, VfsEntryType, std::less<>> VfsDirectory::GetEntries() const {
+    std::map<std::string, VfsEntryType, std::less<>> out;
+    for (const auto& dir : GetSubdirectories())
+        out.emplace(dir->GetName(), VfsEntryType::Directory);
+    for (const auto& file : GetFiles())
+        out.emplace(file->GetName(), VfsEntryType::File);
+    return out;
+}
+
 std::string VfsDirectory::GetFullPath() const {
     if (IsRoot())
         return GetName();
@@ -454,13 +463,41 @@ bool DeepEquals(const VirtualFile& file1, const VirtualFile& file2, std::size_t
     return true;
 }
 
-bool VfsRawCopy(VirtualFile src, VirtualFile dest) {
-    if (src == nullptr || dest == nullptr)
+bool VfsRawCopy(const VirtualFile& src, const VirtualFile& dest, size_t block_size) {
+    if (src == nullptr || dest == nullptr || !src->IsReadable() || !dest->IsWritable())
         return false;
     if (!dest->Resize(src->GetSize()))
         return false;
-    std::vector<u8> data = src->ReadAllBytes();
-    return dest->WriteBytes(data, 0) == data.size();
+
+    std::vector<u8> temp(std::min(block_size, src->GetSize()));
+    for (size_t i = 0; i < src->GetSize(); i += block_size) {
+        const auto read = std::min(block_size, src->GetSize() - i);
+        const auto block = src->Read(temp.data(), read, i);
+
+        if (dest->Write(temp.data(), read, i) != read)
+            return false;
+    }
+
+    return true;
+}
+
+bool VfsRawCopyD(const VirtualDir& src, const VirtualDir& dest, size_t block_size) {
+    if (src == nullptr || dest == nullptr || !src->IsReadable() || !dest->IsWritable())
+        return false;
+
+    for (const auto& file : src->GetFiles()) {
+        const auto out = dest->CreateFile(file->GetName());
+        if (!VfsRawCopy(file, out, block_size))
+            return false;
+    }
+
+    for (const auto& dir : src->GetSubdirectories()) {
+        const auto out = dest->CreateSubdirectory(dir->GetName());
+        if (!VfsRawCopyD(dir, out, block_size))
+            return false;
+    }
+
+    return true;
 }
 
 VirtualDir GetOrCreateDirectoryRelative(const VirtualDir& rel, std::string_view path) {
diff --git a/src/core/file_sys/vfs.h b/src/core/file_sys/vfs.h
index 74489b452..cea4aa8b8 100644
--- a/src/core/file_sys/vfs.h
+++ b/src/core/file_sys/vfs.h
@@ -4,6 +4,7 @@
 
 #pragma once
 
+#include <map>
 #include <memory>
 #include <string>
 #include <string_view>
@@ -265,6 +266,10 @@ public:
     // dest.
     virtual bool Copy(std::string_view src, std::string_view dest);
 
+    // Gets all of the entries directly in the directory (files and dirs), returning a map between
+    // item name -> type.
+    virtual std::map<std::string, VfsEntryType, std::less<>> GetEntries() const;
+
     // Interprets the file with name file instead as a directory of type directory.
     // The directory must have a constructor that takes a single argument of type
     // std::shared_ptr<VfsFile>. Allows to reinterpret container files (i.e NCA, zip, XCI, etc) as a
@@ -311,12 +316,17 @@ public:
 };
 
 // Compare the two files, byte-for-byte, in increments specificed by block_size
-bool DeepEquals(const VirtualFile& file1, const VirtualFile& file2, std::size_t block_size = 0x200);
+bool DeepEquals(const VirtualFile& file1, const VirtualFile& file2, size_t block_size = 0x1000);
 
 // A method that copies the raw data between two different implementations of VirtualFile. If you
 // are using the same implementation, it is probably better to use the Copy method in the parent
 // directory of src/dest.
-bool VfsRawCopy(VirtualFile src, VirtualFile dest);
+bool VfsRawCopy(const VirtualFile& src, const VirtualFile& dest, size_t block_size = 0x1000);
+
+// A method that performs a similar function to VfsRawCopy above, but instead copies entire
+// directories. It suffers the same performance penalties as above and an implementation-specific
+// Copy should always be preferred.
+bool VfsRawCopyD(const VirtualDir& src, const VirtualDir& dest, size_t block_size = 0x1000);
 
 // Checks if the directory at path relative to rel exists. If it does, returns that. If it does not
 // it attempts to create it and returns the new dir or nullptr on failure.
diff --git a/src/core/file_sys/vfs_concat.cpp b/src/core/file_sys/vfs_concat.cpp
index dc7a279a9..d9f9911da 100644
--- a/src/core/file_sys/vfs_concat.cpp
+++ b/src/core/file_sys/vfs_concat.cpp
@@ -5,10 +5,23 @@
 #include <algorithm>
 #include <utility>
 
+#include "common/assert.h"
 #include "core/file_sys/vfs_concat.h"
 
 namespace FileSys {
 
+static bool VerifyConcatenationMapContinuity(const std::map<u64, VirtualFile>& map) {
+    const auto last_valid = --map.end();
+    for (auto iter = map.begin(); iter != last_valid;) {
+        const auto old = iter++;
+        if (old->first + old->second->GetSize() != iter->first) {
+            return false;
+        }
+    }
+
+    return map.begin()->first == 0;
+}
+
 VirtualFile ConcatenateFiles(std::vector<VirtualFile> files, std::string name) {
     if (files.empty())
         return nullptr;
@@ -27,6 +40,11 @@ ConcatenatedVfsFile::ConcatenatedVfsFile(std::vector<VirtualFile> files_, std::s
     }
 }
 
+ConcatenatedVfsFile::ConcatenatedVfsFile(std::map<u64, VirtualFile> files_, std::string name)
+    : files(std::move(files_)), name(std::move(name)) {
+    ASSERT(VerifyConcatenationMapContinuity(files));
+}
+
 ConcatenatedVfsFile::~ConcatenatedVfsFile() = default;
 
 std::string ConcatenatedVfsFile::GetName() const {
@@ -62,7 +80,7 @@ bool ConcatenatedVfsFile::IsReadable() const {
 }
 
 std::size_t ConcatenatedVfsFile::Read(u8* data, std::size_t length, std::size_t offset) const {
-    auto entry = files.end();
+    auto entry = --files.end();
     for (auto iter = files.begin(); iter != files.end(); ++iter) {
         if (iter->first > offset) {
             entry = --iter;
@@ -70,20 +88,17 @@ std::size_t ConcatenatedVfsFile::Read(u8* data, std::size_t length, std::size_t
         }
     }
 
-    // Check if the entry should be the last one. The loop above will make it end().
-    if (entry == files.end() && offset < files.rbegin()->first + files.rbegin()->second->GetSize())
-        --entry;
-
-    if (entry == files.end())
+    if (entry->first + entry->second->GetSize() <= offset)
         return 0;
 
-    const auto remaining = entry->second->GetSize() + offset - entry->first;
-    if (length > remaining) {
-        return entry->second->Read(data, remaining, offset - entry->first) +
-               Read(data + remaining, length - remaining, offset + remaining);
+    const auto read_in =
+        std::min<u64>(entry->first + entry->second->GetSize() - offset, entry->second->GetSize());
+    if (length > read_in) {
+        return entry->second->Read(data, read_in, offset - entry->first) +
+               Read(data + read_in, length - read_in, offset + read_in);
     }
 
-    return entry->second->Read(data, length, offset - entry->first);
+    return entry->second->Read(data, std::min<u64>(read_in, length), offset - entry->first);
 }
 
 std::size_t ConcatenatedVfsFile::Write(const u8* data, std::size_t length, std::size_t offset) {
@@ -93,4 +108,5 @@ std::size_t ConcatenatedVfsFile::Write(const u8* data, std::size_t length, std::
 bool ConcatenatedVfsFile::Rename(std::string_view name) {
     return false;
 }
+
 } // namespace FileSys
diff --git a/src/core/file_sys/vfs_concat.h b/src/core/file_sys/vfs_concat.h
index 717d04bdc..76211d38a 100644
--- a/src/core/file_sys/vfs_concat.h
+++ b/src/core/file_sys/vfs_concat.h
@@ -4,22 +4,25 @@
 
 #pragma once
 
+#include <map>
 #include <memory>
 #include <string_view>
 #include <boost/container/flat_map.hpp>
 #include "core/file_sys/vfs.h"
+#include "core/file_sys/vfs_static.h"
 
 namespace FileSys {
 
-// Wrapper function to allow for more efficient handling of files.size() == 0, 1 cases.
-VirtualFile ConcatenateFiles(std::vector<VirtualFile> files, std::string name = "");
-
 // Class that wraps multiple vfs files and concatenates them, making reads seamless. Currently
 // read-only.
 class ConcatenatedVfsFile : public VfsFile {
     friend VirtualFile ConcatenateFiles(std::vector<VirtualFile> files, std::string name);
 
+    template <u8 filler_byte>
+    friend VirtualFile ConcatenateFiles(std::map<u64, VirtualFile> files, std::string name);
+
     ConcatenatedVfsFile(std::vector<VirtualFile> files, std::string name);
+    ConcatenatedVfsFile(std::map<u64, VirtualFile> files, std::string name);
 
 public:
     ~ConcatenatedVfsFile() override;
@@ -36,8 +39,37 @@ public:
 
 private:
     // Maps starting offset to file -- more efficient.
-    boost::container::flat_map<u64, VirtualFile> files;
+    std::map<u64, VirtualFile> files;
     std::string name;
 };
 
+// Wrapper function to allow for more efficient handling of files.size() == 0, 1 cases.
+VirtualFile ConcatenateFiles(std::vector<VirtualFile> files, std::string name);
+
+// Convenience function that turns a map of offsets to files into a concatenated file, filling gaps
+// with template parameter.
+template <u8 filler_byte>
+VirtualFile ConcatenateFiles(std::map<u64, VirtualFile> files, std::string name) {
+    if (files.empty())
+        return nullptr;
+    if (files.size() == 1)
+        return files.begin()->second;
+
+    const auto last_valid = --files.end();
+    for (auto iter = files.begin(); iter != last_valid;) {
+        const auto old = iter++;
+        if (old->first + old->second->GetSize() != iter->first) {
+            files.emplace(old->first + old->second->GetSize(),
+                          std::make_shared<StaticVfsFile<filler_byte>>(iter->first - old->first -
+                                                                       old->second->GetSize()));
+        }
+    }
+
+    // Ensure the map starts at offset 0 (start of file), otherwise pad to fill.
+    if (files.begin()->first != 0)
+        files.emplace(0, std::make_shared<StaticVfsFile<filler_byte>>(files.begin()->first));
+
+    return std::shared_ptr<VfsFile>(new ConcatenatedVfsFile(std::move(files), std::move(name)));
+}
+
 } // namespace FileSys
diff --git a/src/core/file_sys/vfs_layered.cpp b/src/core/file_sys/vfs_layered.cpp
new file mode 100644
index 000000000..45563d7ae
--- /dev/null
+++ b/src/core/file_sys/vfs_layered.cpp
@@ -0,0 +1,131 @@
+// Copyright 2018 yuzu emulator team
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <algorithm>
+#include <utility>
+#include "core/file_sys/vfs_layered.h"
+
+namespace FileSys {
+
+VirtualDir LayerDirectories(std::vector<VirtualDir> dirs, std::string name) {
+    if (dirs.empty())
+        return nullptr;
+    if (dirs.size() == 1)
+        return dirs[0];
+
+    return std::shared_ptr<VfsDirectory>(new LayeredVfsDirectory(std::move(dirs), std::move(name)));
+}
+
+LayeredVfsDirectory::LayeredVfsDirectory(std::vector<VirtualDir> dirs, std::string name)
+    : dirs(std::move(dirs)), name(std::move(name)) {}
+
+LayeredVfsDirectory::~LayeredVfsDirectory() = default;
+
+std::shared_ptr<VfsFile> LayeredVfsDirectory::GetFileRelative(std::string_view path) const {
+    for (const auto& layer : dirs) {
+        const auto file = layer->GetFileRelative(path);
+        if (file != nullptr)
+            return file;
+    }
+
+    return nullptr;
+}
+
+std::shared_ptr<VfsDirectory> LayeredVfsDirectory::GetDirectoryRelative(
+    std::string_view path) const {
+    std::vector<VirtualDir> out;
+    for (const auto& layer : dirs) {
+        auto dir = layer->GetDirectoryRelative(path);
+        if (dir != nullptr)
+            out.push_back(std::move(dir));
+    }
+
+    return LayerDirectories(std::move(out));
+}
+
+std::shared_ptr<VfsFile> LayeredVfsDirectory::GetFile(std::string_view name) const {
+    return GetFileRelative(name);
+}
+
+std::shared_ptr<VfsDirectory> LayeredVfsDirectory::GetSubdirectory(std::string_view name) const {
+    return GetDirectoryRelative(name);
+}
+
+std::string LayeredVfsDirectory::GetFullPath() const {
+    return dirs[0]->GetFullPath();
+}
+
+std::vector<std::shared_ptr<VfsFile>> LayeredVfsDirectory::GetFiles() const {
+    std::vector<VirtualFile> out;
+    for (const auto& layer : dirs) {
+        for (const auto& file : layer->GetFiles()) {
+            if (std::find_if(out.begin(), out.end(), [&file](const VirtualFile& comp) {
+                    return comp->GetName() == file->GetName();
+                }) == out.end()) {
+                out.push_back(file);
+            }
+        }
+    }
+
+    return out;
+}
+
+std::vector<std::shared_ptr<VfsDirectory>> LayeredVfsDirectory::GetSubdirectories() const {
+    std::vector<std::string> names;
+    for (const auto& layer : dirs) {
+        for (const auto& sd : layer->GetSubdirectories()) {
+            if (std::find(names.begin(), names.end(), sd->GetName()) == names.end())
+                names.push_back(sd->GetName());
+        }
+    }
+
+    std::vector<VirtualDir> out;
+    out.reserve(names.size());
+    for (const auto& subdir : names)
+        out.push_back(GetSubdirectory(subdir));
+
+    return out;
+}
+
+bool LayeredVfsDirectory::IsWritable() const {
+    return false;
+}
+
+bool LayeredVfsDirectory::IsReadable() const {
+    return true;
+}
+
+std::string LayeredVfsDirectory::GetName() const {
+    return name.empty() ? dirs[0]->GetName() : name;
+}
+
+std::shared_ptr<VfsDirectory> LayeredVfsDirectory::GetParentDirectory() const {
+    return dirs[0]->GetParentDirectory();
+}
+
+std::shared_ptr<VfsDirectory> LayeredVfsDirectory::CreateSubdirectory(std::string_view name) {
+    return nullptr;
+}
+
+std::shared_ptr<VfsFile> LayeredVfsDirectory::CreateFile(std::string_view name) {
+    return nullptr;
+}
+
+bool LayeredVfsDirectory::DeleteSubdirectory(std::string_view name) {
+    return false;
+}
+
+bool LayeredVfsDirectory::DeleteFile(std::string_view name) {
+    return false;
+}
+
+bool LayeredVfsDirectory::Rename(std::string_view name_) {
+    name = name_;
+    return true;
+}
+
+bool LayeredVfsDirectory::ReplaceFileWithSubdirectory(VirtualFile file, VirtualDir dir) {
+    return false;
+}
+} // namespace FileSys
diff --git a/src/core/file_sys/vfs_layered.h b/src/core/file_sys/vfs_layered.h
new file mode 100644
index 000000000..4f6e341ab
--- /dev/null
+++ b/src/core/file_sys/vfs_layered.h
@@ -0,0 +1,52 @@
+// Copyright 2018 yuzu emulator team
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <memory>
+#include "core/file_sys/vfs.h"
+
+namespace FileSys {
+
+// Wrapper function to allow for more efficient handling of dirs.size() == 0, 1 cases.
+VirtualDir LayerDirectories(std::vector<VirtualDir> dirs, std::string name = "");
+
+// Class that stacks multiple VfsDirectories on top of each other, attempting to read from the first
+// one and falling back to the one after. The highest priority directory (overwrites all others)
+// should be element 0 in the dirs vector.
+class LayeredVfsDirectory : public VfsDirectory {
+    friend VirtualDir LayerDirectories(std::vector<VirtualDir> dirs, std::string name);
+
+    LayeredVfsDirectory(std::vector<VirtualDir> dirs, std::string name);
+
+public:
+    ~LayeredVfsDirectory() override;
+
+    std::shared_ptr<VfsFile> GetFileRelative(std::string_view path) const override;
+    std::shared_ptr<VfsDirectory> GetDirectoryRelative(std::string_view path) const override;
+    std::shared_ptr<VfsFile> GetFile(std::string_view name) const override;
+    std::shared_ptr<VfsDirectory> GetSubdirectory(std::string_view name) const override;
+    std::string GetFullPath() const override;
+
+    std::vector<std::shared_ptr<VfsFile>> GetFiles() const override;
+    std::vector<std::shared_ptr<VfsDirectory>> GetSubdirectories() const override;
+    bool IsWritable() const override;
+    bool IsReadable() const override;
+    std::string GetName() const override;
+    std::shared_ptr<VfsDirectory> GetParentDirectory() const override;
+    std::shared_ptr<VfsDirectory> CreateSubdirectory(std::string_view name) override;
+    std::shared_ptr<VfsFile> CreateFile(std::string_view name) override;
+    bool DeleteSubdirectory(std::string_view name) override;
+    bool DeleteFile(std::string_view name) override;
+    bool Rename(std::string_view name) override;
+
+protected:
+    bool ReplaceFileWithSubdirectory(VirtualFile file, VirtualDir dir) override;
+
+private:
+    std::vector<VirtualDir> dirs;
+    std::string name;
+};
+
+} // namespace FileSys
diff --git a/src/core/file_sys/vfs_real.cpp b/src/core/file_sys/vfs_real.cpp
index 5e242e20f..9defad04c 100644
--- a/src/core/file_sys/vfs_real.cpp
+++ b/src/core/file_sys/vfs_real.cpp
@@ -413,6 +413,23 @@ std::string RealVfsDirectory::GetFullPath() const {
     return out;
 }
 
+std::map<std::string, VfsEntryType, std::less<>> RealVfsDirectory::GetEntries() const {
+    if (perms == Mode::Append)
+        return {};
+
+    std::map<std::string, VfsEntryType, std::less<>> out;
+    FileUtil::ForeachDirectoryEntry(
+        nullptr, path,
+        [&out](u64* entries_out, const std::string& directory, const std::string& filename) {
+            const std::string full_path = directory + DIR_SEP + filename;
+            out.emplace(filename, FileUtil::IsDirectory(full_path) ? VfsEntryType::Directory
+                                                                   : VfsEntryType::File);
+            return true;
+        });
+
+    return out;
+}
+
 bool RealVfsDirectory::ReplaceFileWithSubdirectory(VirtualFile file, VirtualDir dir) {
     return false;
 }
diff --git a/src/core/file_sys/vfs_real.h b/src/core/file_sys/vfs_real.h
index 681c43e82..5b61db90d 100644
--- a/src/core/file_sys/vfs_real.h
+++ b/src/core/file_sys/vfs_real.h
@@ -98,6 +98,7 @@ public:
     bool DeleteFile(std::string_view name) override;
     bool Rename(std::string_view name) override;
     std::string GetFullPath() const override;
+    std::map<std::string, VfsEntryType, std::less<>> GetEntries() const override;
 
 protected:
     bool ReplaceFileWithSubdirectory(VirtualFile file, VirtualDir dir) override;
diff --git a/src/core/file_sys/vfs_static.h b/src/core/file_sys/vfs_static.h
new file mode 100644
index 000000000..4dd47ffcc
--- /dev/null
+++ b/src/core/file_sys/vfs_static.h
@@ -0,0 +1,78 @@
+// Copyright 2018 yuzu emulator team
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <algorithm>
+#include <memory>
+#include <string_view>
+
+#include "core/file_sys/vfs.h"
+
+namespace FileSys {
+
+template <u8 value>
+class StaticVfsFile : public VfsFile {
+public:
+    explicit StaticVfsFile(size_t size = 0, std::string name = "", VirtualDir parent = nullptr)
+        : size(size), name(std::move(name)), parent(std::move(parent)) {}
+
+    std::string GetName() const override {
+        return name;
+    }
+
+    size_t GetSize() const override {
+        return size;
+    }
+
+    bool Resize(size_t new_size) override {
+        size = new_size;
+        return true;
+    }
+
+    std::shared_ptr<VfsDirectory> GetContainingDirectory() const override {
+        return parent;
+    }
+
+    bool IsWritable() const override {
+        return false;
+    }
+
+    bool IsReadable() const override {
+        return true;
+    }
+
+    size_t Read(u8* data, size_t length, size_t offset) const override {
+        const auto read = std::min(length, size - offset);
+        std::fill(data, data + read, value);
+        return read;
+    }
+
+    size_t Write(const u8* data, size_t length, size_t offset) override {
+        return 0;
+    }
+
+    boost::optional<u8> ReadByte(size_t offset) const override {
+        if (offset < size)
+            return value;
+        return boost::none;
+    }
+
+    std::vector<u8> ReadBytes(size_t length, size_t offset) const override {
+        const auto read = std::min(length, size - offset);
+        return std::vector<u8>(read, value);
+    }
+
+    bool Rename(std::string_view new_name) override {
+        name = new_name;
+        return true;
+    }
+
+private:
+    size_t size;
+    std::string name;
+    VirtualDir parent;
+};
+
+} // namespace FileSys
diff --git a/src/core/file_sys/vfs_vector.cpp b/src/core/file_sys/vfs_vector.cpp
index ec7f735b5..7033e2c88 100644
--- a/src/core/file_sys/vfs_vector.cpp
+++ b/src/core/file_sys/vfs_vector.cpp
@@ -3,10 +3,64 @@
 // Refer to the license.txt file included.
 
 #include <algorithm>
+#include <cstring>
 #include <utility>
 #include "core/file_sys/vfs_vector.h"
 
 namespace FileSys {
+VectorVfsFile::VectorVfsFile(std::vector<u8> initial_data, std::string name, VirtualDir parent)
+    : data(std::move(initial_data)), name(std::move(name)), parent(std::move(parent)) {}
+
+VectorVfsFile::~VectorVfsFile() = default;
+
+std::string VectorVfsFile::GetName() const {
+    return name;
+}
+
+size_t VectorVfsFile::GetSize() const {
+    return data.size();
+}
+
+bool VectorVfsFile::Resize(size_t new_size) {
+    data.resize(new_size);
+    return true;
+}
+
+std::shared_ptr<VfsDirectory> VectorVfsFile::GetContainingDirectory() const {
+    return parent;
+}
+
+bool VectorVfsFile::IsWritable() const {
+    return true;
+}
+
+bool VectorVfsFile::IsReadable() const {
+    return true;
+}
+
+size_t VectorVfsFile::Read(u8* data_, size_t length, size_t offset) const {
+    const auto read = std::min(length, data.size() - offset);
+    std::memcpy(data_, data.data() + offset, read);
+    return read;
+}
+
+size_t VectorVfsFile::Write(const u8* data_, size_t length, size_t offset) {
+    if (offset + length > data.size())
+        data.resize(offset + length);
+    const auto write = std::min(length, data.size() - offset);
+    std::memcpy(data.data(), data_, write);
+    return write;
+}
+
+bool VectorVfsFile::Rename(std::string_view name_) {
+    name = name_;
+    return true;
+}
+
+void VectorVfsFile::Assign(std::vector<u8> new_data) {
+    data = std::move(new_data);
+}
+
 VectorVfsDirectory::VectorVfsDirectory(std::vector<VirtualFile> files_,
                                        std::vector<VirtualDir> dirs_, std::string name_,
                                        VirtualDir parent_)
diff --git a/src/core/file_sys/vfs_vector.h b/src/core/file_sys/vfs_vector.h
index cba44a7a6..115c3ae95 100644
--- a/src/core/file_sys/vfs_vector.h
+++ b/src/core/file_sys/vfs_vector.h
@@ -8,6 +8,31 @@
 
 namespace FileSys {
 
+// An implementation of VfsFile that is backed by a vector optionally supplied upon construction
+class VectorVfsFile : public VfsFile {
+public:
+    explicit VectorVfsFile(std::vector<u8> initial_data = {}, std::string name = "",
+                           VirtualDir parent = nullptr);
+    ~VectorVfsFile() override;
+
+    std::string GetName() const override;
+    size_t GetSize() const override;
+    bool Resize(size_t new_size) override;
+    std::shared_ptr<VfsDirectory> GetContainingDirectory() const override;
+    bool IsWritable() const override;
+    bool IsReadable() const override;
+    size_t Read(u8* data, size_t length, size_t offset) const override;
+    size_t Write(const u8* data, size_t length, size_t offset) override;
+    bool Rename(std::string_view name) override;
+
+    virtual void Assign(std::vector<u8> new_data);
+
+private:
+    std::vector<u8> data;
+    VirtualDir parent;
+    std::string name;
+};
+
 // An implementation of VfsDirectory that maintains two vectors for subdirectories and files.
 // Vector data is supplied upon construction.
 class VectorVfsDirectory : public VfsDirectory {
diff --git a/src/core/hle/service/filesystem/filesystem.cpp b/src/core/hle/service/filesystem/filesystem.cpp
index d349ee686..aed2abb71 100644
--- a/src/core/hle/service/filesystem/filesystem.cpp
+++ b/src/core/hle/service/filesystem/filesystem.cpp
@@ -343,6 +343,15 @@ std::shared_ptr<FileSys::RegisteredCache> GetSDMCContents() {
     return sdmc_factory->GetSDMCContents();
 }
 
+FileSys::VirtualDir GetModificationLoadRoot(u64 title_id) {
+    LOG_TRACE(Service_FS, "Opening mod load root for tid={:016X}", title_id);
+
+    if (bis_factory == nullptr)
+        return nullptr;
+
+    return bis_factory->GetModificationLoadRoot(title_id);
+}
+
 void CreateFactories(const FileSys::VirtualFilesystem& vfs, bool overwrite) {
     if (overwrite) {
         bis_factory = nullptr;
@@ -354,9 +363,11 @@ void CreateFactories(const FileSys::VirtualFilesystem& vfs, bool overwrite) {
                                              FileSys::Mode::ReadWrite);
     auto sd_directory = vfs->OpenDirectory(FileUtil::GetUserPath(FileUtil::UserPath::SDMCDir),
                                            FileSys::Mode::ReadWrite);
+    auto load_directory = vfs->OpenDirectory(FileUtil::GetUserPath(FileUtil::UserPath::LoadDir),
+                                             FileSys::Mode::ReadWrite);
 
     if (bis_factory == nullptr)
-        bis_factory = std::make_unique<FileSys::BISFactory>(nand_directory);
+        bis_factory = std::make_unique<FileSys::BISFactory>(nand_directory, load_directory);
     if (save_data_factory == nullptr)
         save_data_factory = std::make_unique<FileSys::SaveDataFactory>(std::move(nand_directory));
     if (sdmc_factory == nullptr)
diff --git a/src/core/hle/service/filesystem/filesystem.h b/src/core/hle/service/filesystem/filesystem.h
index aab65a2b8..7039a2247 100644
--- a/src/core/hle/service/filesystem/filesystem.h
+++ b/src/core/hle/service/filesystem/filesystem.h
@@ -52,6 +52,8 @@ std::shared_ptr<FileSys::RegisteredCache> GetSystemNANDContents();
 std::shared_ptr<FileSys::RegisteredCache> GetUserNANDContents();
 std::shared_ptr<FileSys::RegisteredCache> GetSDMCContents();
 
+FileSys::VirtualDir GetModificationLoadRoot(u64 title_id);
+
 // Creates the SaveData, SDMC, and BIS Factories. Should be called once and before any function
 // above is called.
 void CreateFactories(const FileSys::VirtualFilesystem& vfs, bool overwrite = true);