Add and test a memory::translate_addr function

src/main.rs

@@ -11,8 +11,8 @@ entry_point!(kernel_main);
 #[cfg(not(test))]
 fn kernel_main(boot_info: &'static BootInfo) -> ! {
     use blog_os::interrupts::PICS;
-    use blog_os::memory::active_level_4_table;
-    use x86_64::{structures::paging::PageTable, VirtAddr};
+    use blog_os::memory::translate_addr;
+    use x86_64::VirtAddr;
 
     println!("Hello World{}", "!");
 
@@ -21,24 +21,21 @@ fn kernel_main(boot_info: &'static BootInfo) -> ! {
     unsafe { PICS.lock().initialize() };
     x86_64::instructions::interrupts::enable();
 
-    let l4_table = unsafe { active_level_4_table(boot_info.physical_memory_offset) };
-    for (i, entry) in l4_table.iter().enumerate() {
-        if !entry.is_unused() {
-            println!("L4 Entry {}: {:?}", i, entry);
+    let addresses = [
+        // the identity-mapped vga buffer page
+        0xb8000,
+        // some code page
+        0x20010a,
+        // some stack page
+        0x57ac_001f_fe48,
+        // virtual address mapped to physical address 0
+        boot_info.physical_memory_offset,
+    ];
 
-            // get the physical address from the entry and convert it
-            let phys = entry.frame().unwrap().start_address();
-            let virt = phys.as_u64() + boot_info.physical_memory_offset;
-            let ptr = VirtAddr::new(virt).as_mut_ptr();
-            let l3_table: &PageTable = unsafe { &*ptr };
-
-            // print non-empty entries of the level 3 table
-            for (i, entry) in l3_table.iter().enumerate() {
-                if !entry.is_unused() {
-                    println!("  L3 Entry {}: {:?}", i, entry);
-                }
-            }
-        }
+    for &address in &addresses {
+        let virt = VirtAddr::new(address);
+        let phys = unsafe { translate_addr(virt, boot_info.physical_memory_offset) };
+        println!("{:?} -> {:?}", virt, phys);
     }
 
     println!("It did not crash!");

src/memory.rs

@@ -1,4 +1,4 @@
-use x86_64::structures::paging::PageTable;
+use x86_64::{structures::paging::PageTable, PhysAddr, VirtAddr};
 
 /// Returns a mutable reference to the active level 4 table.
 ///
@@ -17,3 +17,53 @@ pub unsafe fn active_level_4_table(physical_memory_offset: u64) -> &'static mut
 
     &mut *page_table_ptr // unsafe
 }
+
+/// Translates the given virtual address to the mapped physical address, or
+/// `None` if the address is not mapped.
+///
+/// This function is unsafe because the caller must guarantee that the
+/// complete physical memory is mapped to virtual memory at the passed
+/// `physical_memory_offset`.
+pub unsafe fn translate_addr(addr: VirtAddr, physical_memory_offset: u64) -> Option<PhysAddr> {
+    translate_addr_inner(addr, physical_memory_offset)
+}
+
+/// Private function that is called by `translate_addr`.
+///
+/// This function is safe to limit the scope of `unsafe` because Rust treats
+/// the whole body of unsafe functions as an unsafe block. This function must
+/// only be reachable through `unsafe fn` from outside of this module.
+fn translate_addr_inner(addr: VirtAddr, physical_memory_offset: u64) -> 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 = frame.start_address().as_u64() + physical_memory_offset;
+        let table_ptr: *const PageTable = VirtAddr::new(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()))
+}