Add switch method to kernel

3 files changed, 38 insertions, 4 deletions

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
@@ -16,13 +16,20 @@ namespace teachos::arch::memory::paging
      * @brief Constructor.
      *
      * @param frame Frame that should be mapped as the level 4 page table.
+     */
+    inactive_page_table(allocator::physical_frame frame);
+
+    /**
+     * @brief Constructor.
+     *
+     * @param frame Frame that should be mapped as the level 4 page table.
      * @param active_page_table Actual active page table that should be unmapped so we can map a new level 4
      * page table.
      * @param temporary_page Temporary page that should be used to map the given frame as the new level 4 page
      * 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:
@@ -84,6 +88,23 @@ namespace teachos::arch::memory::paging
     }
 
     /**
+     * @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);