From 15d14ed67f7ae0392e6f14cd3953da5aa3fc4176 Mon Sep 17 00:00:00 2001
From: Yash Karandikar <nerdstep710@gmail.com>
Date: Sun, 19 Sep 2021 13:29:37 -0500
Subject: [PATCH] Add support for translating virtual addresses to physical
 ones

---
 src/main.rs   | 23 +++++++++++++++++++++++
 src/memory.rs | 18 ++++++++++++++++++
 2 files changed, 41 insertions(+)
 create mode 100644 src/memory.rs

diff --git a/src/main.rs b/src/main.rs
index 530cf33..4dd62e9 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -10,6 +10,8 @@ mod gdt;
 mod interrupts;
 mod shell;
 mod vga_buffer;
+mod memory;
+
 use core::panic::PanicInfo;
 use bootloader::BootInfo;
 
@@ -27,6 +29,7 @@ fn init() {
 #[no_mangle]
 pub extern "C" fn _start(boot_info: &'static BootInfo) {
     use crate::vga_buffer::{change_color, Color};
+    use x86_64::{VirtAddr, structures::paging::Translate};
 
     init();
     print!("[ ");
@@ -35,6 +38,26 @@ pub extern "C" fn _start(boot_info: &'static BootInfo) {
     change_color(Color::White, Color::Black);
     println!(" ] Initialized GDT and interrupts");
 
+    let phys_mem_offset = VirtAddr::new(boot_info.physical_memory_offset);
+    let mapper = unsafe { memory::init(phys_mem_offset) };
+
+    let addresses = [
+        // the identity-mapped vga buffer page
+        0xb8000,
+        // some code page
+        0x201008,
+        // some stack page
+        0x0100_0020_1a10,
+        // virtual address mapped to physical address 0
+        boot_info.physical_memory_offset,
+    ];
+
+    for &address in &addresses {
+        let virt = VirtAddr::new(address);
+        let phys = mapper.translate_addr(virt);
+        println!("{:?} -> {:?}", virt, phys);        
+    }
+
     print!("Welcome to ");
     change_color(Color::Blue, Color::Black);
     println!("KarxOS!");
diff --git a/src/memory.rs b/src/memory.rs
new file mode 100644
index 0000000..7a86c6a
--- /dev/null
+++ b/src/memory.rs
@@ -0,0 +1,18 @@
+use x86_64::{VirtAddr, structures::paging::{PageTable, OffsetPageTable}};
+
+pub unsafe fn init(physical_memory_offset: VirtAddr) -> OffsetPageTable<'static> {
+    let level_4_table = active_level_4_table(physical_memory_offset);
+    OffsetPageTable::new(level_4_table, physical_memory_offset)
+}
+
+unsafe fn active_level_4_table(physical_memory_offset: VirtAddr) -> &'static mut PageTable {
+    use x86_64::registers::control::Cr3;
+
+    let (level_4_table_frame, _) = Cr3::read();
+
+    let phys = level_4_table_frame.start_address();
+    let virt = physical_memory_offset + phys.as_u64();
+    let page_table_ptr: *mut PageTable = virt.as_mut_ptr();
+
+    &mut *page_table_ptr
+}