From 9a185c1533bd2197d0e830369b4cc26abf88e2c9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Matteo=20Gm=C3=BCr?= <matteo.gmuer1@ost.ch>
Date: Fri, 11 Apr 2025 09:25:04 +0000
Subject: Document methods and move them into kernel cpu folder

---
 arch/x86_64/src/context_switching/main.cpp | 63 +++++++-----------------------
 1 file changed, 14 insertions(+), 49 deletions(-)

(limited to 'arch/x86_64/src/context_switching/main.cpp')

diff --git a/arch/x86_64/src/context_switching/main.cpp b/arch/x86_64/src/context_switching/main.cpp
index 0f2ec93..5cac878 100644
--- a/arch/x86_64/src/context_switching/main.cpp
+++ b/arch/x86_64/src/context_switching/main.cpp
@@ -12,46 +12,6 @@ namespace teachos::arch::context_switching
 {
   namespace
   {
-
-    /**
-     * @brief Switch context into the mode defined in the segment selectors.
-     *
-     * Setup the stack IRETQ expects to switch the mode:
-     * 1. push data selector
-     * 2. push current stack pointer
-     * 3. push eflags
-     * 4. push code segment selector
-     * 5. push return address
-     *
-     * @param data_segment
-     * @param code_segment
-     * @param address
-     */
-    [[gnu::naked]]
-    auto far_return(context_switching::interrupt_descriptor_table::segment_selector data_segment,
-                    context_switching::interrupt_descriptor_table::segment_selector code_segment, uint64_t address)
-        -> void
-    {
-      asm volatile("mov %[data_segment], %%rax\n"
-                   "mov %%rax, %%ds\n"
-                   "mov %%rax, %%es\n"
-                   "mov %%rax, %%fs\n"
-                   "mov %%rax, %%gs\n"
-                   "mov %%rsp, %%rax\n"
-
-                   "push %[data_segment]\n"
-                   "push %%rax\n"
-                   "pushfq\n"
-                   "push %[code_segment]\n"
-                   "mov %[return_function], %%rax\n"
-                   "push %%rax\n"
-
-                   "iretq\n"
-                   :
-                   : [data_segment] "m"(data_segment), [code_segment] "m"(code_segment), [return_function] "r"(address)
-                   : "rax");
-    }
-
     constexpr context_switching::interrupt_descriptor_table::segment_selector USER_CODE_SEGMENT_SELECTOR{
         3U, context_switching::interrupt_descriptor_table::segment_selector::REQUEST_LEVEL_USER};
     constexpr context_switching::interrupt_descriptor_table::segment_selector USER_DATA_SEGMENT_SELECTOR{
@@ -61,6 +21,14 @@ namespace teachos::arch::context_switching
 
   auto initialize_descriptor_tables() -> descriptor_tables
   {
+    static bool initalized = false;
+    if (initalized)
+    {
+      descriptor_tables tables = {segment_descriptor_table::get_or_create_global_descriptor_table(),
+                                  interrupt_descriptor_table::get_or_create_interrupt_descriptor_table()};
+      return tables;
+    }
+
     kernel::cpu::clear_interrupt_flag();
 
     segment_descriptor_table::update_global_descriptor_table_register();
@@ -68,7 +36,7 @@ namespace teachos::arch::context_switching
 
     interrupt_descriptor_table::segment_selector segment_selector{
         1U, interrupt_descriptor_table::segment_selector::REQUEST_LEVEL_KERNEL};
-    kernel::cpu::far_pointer pointer{&kernel::cpu::reload_segment_registers, segment_selector};
+    kernel::cpu::far_pointer pointer{&kernel::cpu::reload_data_segment_registers, segment_selector};
     kernel::cpu::call(pointer);
 
     segment_descriptor_table::update_task_state_segment_register();
@@ -77,17 +45,13 @@ namespace teachos::arch::context_switching
 
     descriptor_tables tables = {segment_descriptor_table::get_or_create_global_descriptor_table(),
                                 interrupt_descriptor_table::get_or_create_interrupt_descriptor_table()};
+    initalized = true;
     return tables;
   }
 
   auto user_mode_main() -> void
   {
-    auto current_segment = kernel::cpu::read_code_segment_register();
-    exception_handling::assert(USER_CODE_SEGMENT_SELECTOR == current_segment,
-                               "[Context Switching] Context switch into user mode not successful");
-    exception_handling::assert(USER_DATA_SEGMENT_SELECTOR == kernel::cpu::validate_data_segment_registers(),
-                               "[Context Switching] Context switch into user mode not successful");
-
+    kernel::cpu::validate_segment_registers(USER_DATA_SEGMENT_SELECTOR, USER_CODE_SEGMENT_SELECTOR);
     video::vga::text::write("Successfully entered user mode!", video::vga::text::common_attributes::green_on_black);
   }
 
@@ -99,8 +63,9 @@ namespace teachos::arch::context_switching
   auto switch_context(interrupt_descriptor_table::segment_selector data_segment,
                       interrupt_descriptor_table::segment_selector code_segment, void (*return_function)()) -> void
   {
-    kernel::cpu::set_segment_registers(data_segment);
-    far_return(data_segment, code_segment, reinterpret_cast<uint64_t>(return_function));
+    (void)initialize_descriptor_tables();
+    kernel::cpu::set_data_segment_registers(data_segment);
+    kernel::cpu::set_code_segment_register(data_segment, code_segment, reinterpret_cast<uint64_t>(return_function));
   }
 
 }  // namespace teachos::arch::context_switching
-- 
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