From d18cf4a3e4abdea80992b8bba3d1ca50ae215253 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Matteo=20Gm=C3=BCr?= <matteo.gmuer1@ost.ch>
Date: Sat, 9 Nov 2024 14:51:10 +0000
Subject: Add switch method to kernel

---
 .../arch/memory/paging/inactive_page_table.hpp     |  9 +++++++-
 .../include/arch/memory/paging/kernel_mapper.hpp   | 25 ++++++++++++++++++++--
 .../src/memory/paging/inactive_page_table.cpp      |  8 ++++++-
 3 files changed, 38 insertions(+), 4 deletions(-)

(limited to 'arch')

diff --git a/arch/x86_64/include/arch/memory/paging/inactive_page_table.hpp b/arch/x86_64/include/arch/memory/paging/inactive_page_table.hpp
index 54a53f4..a9ab258 100644
--- a/arch/x86_64/include/arch/memory/paging/inactive_page_table.hpp
+++ b/arch/x86_64/include/arch/memory/paging/inactive_page_table.hpp
@@ -12,6 +12,13 @@ namespace teachos::arch::memory::paging
    */
   struct inactive_page_table
   {
+    /**
+     * @brief Constructor.
+     *
+     * @param frame Frame that should be mapped as the level 4 page table.
+     */
+    inactive_page_table(allocator::physical_frame frame);
+
     /**
      * @brief Constructor.
      *
@@ -22,7 +29,7 @@ namespace teachos::arch::memory::paging
      * table.
      */
     inactive_page_table(allocator::physical_frame frame, active_page_table & active_page_table,
-                        temporary_page temporary_page);
+                        temporary_page & temporary_page);
 
     allocator::physical_frame page_table_level_4_frame;  ///< Temporary level 4 page table
   };
diff --git a/arch/x86_64/include/arch/memory/paging/kernel_mapper.hpp b/arch/x86_64/include/arch/memory/paging/kernel_mapper.hpp
index 67d1673..0786ec1 100644
--- a/arch/x86_64/include/arch/memory/paging/kernel_mapper.hpp
+++ b/arch/x86_64/include/arch/memory/paging/kernel_mapper.hpp
@@ -44,8 +44,12 @@ namespace teachos::arch::memory::paging
       auto const frame = allocator.allocate_frame();
       exception_handling::assert(frame.has_value(),
                                  "[Kernel Mapper] Frame could not be allocated and therefore kernel not mapped");
-      inactive_page_table inactive_table{frame.value(), active_table, temporary_page};
-      remap_elf_kernel_sections(inactive_table, temporary_page, active_table);
+      inactive_page_table new_table{frame.value(), active_table, temporary_page};
+      remap_elf_kernel_sections(new_table, temporary_page, active_table);
+      auto const old_table = switch_active_page_table(new_table);
+      auto const old_level_4_page =
+          virtual_page::containing_address(old_table.page_table_level_4_frame.start_address());
+      active_table.unmap_page(allocator, old_level_4_page);
     }
 
   private:
@@ -83,6 +87,23 @@ namespace teachos::arch::memory::paging
       temporary_page.unmap_page(active_table);
     }
 
+    /**
+     * @brief Switches the current active table pointed to by the CR3 register with another page table that is currently
+     * inactive.
+     *
+     * @param new_table Inactive page table that should now be made active and replace the current active one.
+     * @return The previous active page table.
+     */
+    auto switch_active_page_table(inactive_page_table new_table) -> inactive_page_table
+    {
+      auto const backup = allocator::physical_frame::containing_address(cpu::read_cr3_register());
+      auto const old_table = inactive_page_table{backup};
+
+      auto const new_address = new_table.page_table_level_4_frame.start_address();
+      cpu::write_cr3_register(new_address);
+      return old_table;
+    }
+
     /**
      * @brief Maps the required entries according to every elf section and it's contained frames. Additionally each of
      * thoose frames gets the correct entry flags according to elf section flags.
diff --git a/arch/x86_64/src/memory/paging/inactive_page_table.cpp b/arch/x86_64/src/memory/paging/inactive_page_table.cpp
index f6ecb8b..4e0610e 100644
--- a/arch/x86_64/src/memory/paging/inactive_page_table.cpp
+++ b/arch/x86_64/src/memory/paging/inactive_page_table.cpp
@@ -2,8 +2,14 @@
 
 namespace teachos::arch::memory::paging
 {
+  inactive_page_table::inactive_page_table(allocator::physical_frame frame)
+      : page_table_level_4_frame{frame}
+  {
+    // Nothing to do
+  }
+
   inactive_page_table::inactive_page_table(allocator::physical_frame frame, active_page_table & active_page_table,
-                                           temporary_page temporary_page)
+                                           temporary_page & temporary_page)
       : page_table_level_4_frame{frame}
   {
     auto table = temporary_page.map_table_frame(page_table_level_4_frame, active_page_table);
-- 
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