paging

Cargo.toml

@@ -12,7 +12,7 @@ name = "stack_overflow"
 harness = false
 
 [dependencies]
-bootloader = "0.9.8"
+bootloader = { version = "0.9.23", features = ["map_physical_memory"]}
 volatile = "0.2.6"
 spin = "0.5.2"
 x86_64 = "0.14.2"

src/lib.rs

@@ -9,9 +9,16 @@ use core::panic::PanicInfo;
 
 pub mod gdt;
 pub mod interrupts;
+pub mod memory;
 pub mod serial;
 pub mod vga_buffer;
 
+#[cfg(test)]
+use bootloader::{entry_point, BootInfo};
+
+#[cfg(test)]
+entry_point!(test_kernel_main);
+
 pub fn init() {
     gdt::init();
     interrupts::init_idt();
@@ -72,8 +79,8 @@ pub fn hlt_loop() -> ! {
 
 /// Entry point for `cargo xtest`
 #[cfg(test)]
-#[no_mangle]
-pub extern "C" fn _start() -> ! {
+fn test_kernel_main(_boot_info: &'static BootInfo) -> ! {
+    // like before
     init();
     test_main();
     hlt_loop();

src/main.rs

@@ -5,21 +5,39 @@
 #![reexport_test_harness_main = "test_main"]
 
 use blog_os::println;
+use bootloader::{entry_point, BootInfo};
 use core::panic::PanicInfo;
 
-#[no_mangle]
-pub extern "C" fn _start() -> ! {
-    use x86_64::registers::control::Cr3;
-    println!("Hello World{}", "!");
+entry_point!(kernel_main);
+
+fn kernel_main(boot_info: &'static BootInfo) -> ! {
+    use blog_os::memory::active_level_4_table;
+    use blog_os::memory::translate_addr;
+    use x86_64::VirtAddr;
 
+    println!("Hello World{}", "!");
     blog_os::init();
 
-    let (level_4_page_table, _) = Cr3::read();
-    println!(
-        "Level 4 page table at: {:?}",
-        level_4_page_table.start_address()
-    );
+    let phys_mem_offset = VirtAddr::new(boot_info.physical_memory_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 = unsafe { translate_addr(virt, phys_mem_offset) };
+        println!("{:?} -> {:?}", virt, phys);
+    }
 
+    // as before
     #[cfg(test)]
     test_main();
 

src/memory.rs

@@ -0,0 +1,52 @@
+use x86_64::{structures::paging::PageTable, PhysAddr, VirtAddr};
+
+pub 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
+}
+
+pub unsafe fn translate_addr(addr: VirtAddr, physical_memory_offset: VirtAddr) -> Option<PhysAddr> {
+    translate_addr_inner(addr, physical_memory_offset)
+}
+
+fn translate_addr_inner(addr: VirtAddr, physical_memory_offset: VirtAddr) -> Option<PhysAddr> {
+    use x86_64::registers::control::Cr3;
+    use x86_64::structures::paging::page_table::FrameError;
+
+    // read the active level 4 frame from the CR3 register
+    let (level_4_table_frame, _) = Cr3::read();
+
+    let table_indexes = [
+        addr.p4_index(),
+        addr.p3_index(),
+        addr.p2_index(),
+        addr.p1_index(),
+    ];
+    let mut frame = level_4_table_frame;
+
+    // traverse the multi-level page table
+    for &index in &table_indexes {
+        // convert the frame into a page table reference
+        let virt = physical_memory_offset + frame.start_address().as_u64();
+        let table_ptr: *const PageTable = virt.as_ptr();
+        let table = unsafe { &*table_ptr };
+
+        // read the page table entry and update `frame`
+        let entry = &table[index];
+        frame = match entry.frame() {
+            Ok(frame) => frame,
+            Err(FrameError::FrameNotPresent) => return None,
+            Err(FrameError::HugeFrame) => panic!("huge pages not supported"),
+        };
+    }
+
+    // calculate the physical address by adding the page offset
+    Some(frame.start_address() + u64::from(addr.page_offset()))
+}