paging

os/Cargo.toml

@@ -12,10 +12,10 @@ spin = "0.9.8"
 x86_64 = "0.14.2"
 volatile = "0.2.6"
 uart_16550 = "0.2.0"
-bootloader = "0.9.23"
 pic8259 = "0.10.4"
 pc-keyboard = "0.7.0"
 lazy_static = { version = "1.0", features = ["spin_no_std"] }
+bootloader = { version = "0.9.23", features = ["map_physical_memory"]}
 
 [package.metadata.bootimage]
 test-success-exit-code = 33

os/src/interrupts.rs

@@ -1,7 +1,7 @@
-use crate::{gdt, print, println};
+use crate::{gdt, print, println, hlt_loop};
 use lazy_static::lazy_static;
 use pic8259::ChainedPics;
-use x86_64::structures::idt::{InterruptDescriptorTable, InterruptStackFrame};
+use x86_64::structures::idt::{InterruptDescriptorTable, InterruptStackFrame, PageFaultErrorCode};
 
 pub const PIC_1_OFFSET: u8 = 32;
 pub const PIC_2_OFFSET: u8 = PIC_1_OFFSET + 8;
@@ -37,6 +37,7 @@ lazy_static! {
                 .set_handler_fn(double_fault_handler)
                 .set_stack_index(gdt::DOUBLE_FAULT_IST_INDEX);
         }
+        idt.page_fault.set_handler_fn(page_fault_handler);
         idt
     };
 }
@@ -102,3 +103,16 @@ extern "x86-interrupt" fn keyboard_interrupt_handler(_stack_frame: InterruptStac
             .notify_end_of_interrupt(InterruptIndex::Keyboard.as_u8());
     }
 }
+
+extern "x86-interrupt" fn page_fault_handler(
+    stack_frame: InterruptStackFrame,
+    error_code: PageFaultErrorCode,
+) {
+    use x86_64::registers::control::Cr2;
+
+    println!("EXCEPTION: PAGE FAULT");
+    println!("Accessed Address: {:?}", Cr2::read());
+    println!("Error Code: {:?}", error_code);
+    println!("{:#?}", stack_frame);
+    hlt_loop();
+}

os/src/lib.rs

@@ -5,10 +5,14 @@
 #![test_runner(crate::test_runner)]
 #![reexport_test_harness_main = "test_main"]
 
+#[cfg(test)]
+use bootloader::{entry_point, BootInfo};
+
 pub mod gdt;
 pub mod interrupts;
 pub mod serial;
 pub mod vga;
+pub mod memory;
 
 pub fn init() {
     gdt::init();
@@ -25,11 +29,14 @@ pub fn hlt_loop() -> ! {
 
 /// Entry point for `cargo test`
 #[cfg(test)]
-#[no_mangle]
-pub extern "C" fn _start() -> ! {
+entry_point!(test_kernel_main);
+
+#[cfg(test)]
+fn test_kernel_main(_boot_info: &'static BootInfo) -> ! {
+    // like before
     init();
     test_main();
-    crate::hlt_loop()
+    hlt_loop();
 }
 
 // Boilerplate for testing

os/src/main.rs

@@ -5,13 +5,34 @@
 #![reexport_test_harness_main = "test_main"]
 
 use knarkos::println;
+use bootloader::{BootInfo, entry_point};
 
-#[no_mangle]
-pub extern "C" fn _start() -> ! {
+entry_point!(kernel_main);
+
+fn kernel_main(boot_info: &'static BootInfo) -> ! {
+    use knarkos::memory;
+    use knarkos::memory::BootInfoFrameAllocator;
+    use x86_64::{structures::paging::Page, VirtAddr}; // new import
+
     // Initialize operating system
     knarkos::init();
     println!("Hello, world!");
 
+    // Read page tables
+    let phys_mem_offset = VirtAddr::new(boot_info.physical_memory_offset);
+    let mut mapper = unsafe { memory::init(phys_mem_offset) };
+    let mut frame_allocator = unsafe {
+        BootInfoFrameAllocator::init(&boot_info.memory_map)
+    };
+
+    // map an unused page
+    let page = Page::containing_address(VirtAddr::new(0));
+    memory::create_example_mapping(page, &mut mapper, &mut frame_allocator);
+
+    // write the string `New!` to the screen through the new mapping
+    let page_ptr: *mut u64 = page.start_address().as_mut_ptr();
+    unsafe { page_ptr.offset(400).write_volatile(0x_f021_f077_f065_f04e)};
+
     #[cfg(test)]
     test_main();
 

os/src/memory.rs

@@ -0,0 +1,97 @@
+use x86_64::{
+    structures::paging::{Page, PhysFrame, Mapper, Size4KiB, FrameAllocator, OffsetPageTable, PageTable},
+    PhysAddr, VirtAddr,
+};
+use bootloader::bootinfo::{MemoryMap, MemoryRegionType};
+
+/// Initialize a new OffsetPageTable.
+///
+/// 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`. Also, this function must be only called once
+/// to avoid aliasing `&mut` references (which is undefined behavior).
+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)
+}
+
+/// Returns a mutable reference to the active level 4 table.
+///
+/// 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`. Also, this function must be only called once
+/// to avoid aliasing `&mut` references (which is undefined behavior).
+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 // unsafe
+}
+
+/// Creates an example mapping for the given page to frame `0xb8000`.
+pub fn create_example_mapping(
+    page: Page,
+    mapper: &mut OffsetPageTable,
+    frame_allocator: &mut impl FrameAllocator<Size4KiB>,
+) {
+    use x86_64::structures::paging::PageTableFlags as Flags;
+
+    let frame = PhysFrame::containing_address(PhysAddr::new(0xb8000));
+    let flags = Flags::PRESENT | Flags::WRITABLE;
+
+    let map_to_result = unsafe {
+        // FIXME: this is not safe, we do it only for testing
+        mapper.map_to(page, frame, flags, frame_allocator)
+    };
+    map_to_result.expect("map_to failed").flush();
+}
+
+/// A FrameAllocator that returns usable frames from the bootloader's memory map.
+pub struct BootInfoFrameAllocator {
+    memory_map: &'static MemoryMap,
+    next: usize,
+}
+
+impl BootInfoFrameAllocator {
+    /// Create a FrameAllocator from the passed memory map.
+    ///
+    /// This function is unsafe because the caller must guarantee that the passed
+    /// memory map is valid. The main requirement is that all frames that are marked
+    /// as `USABLE` in it are really unused.
+    pub unsafe fn init(memory_map: &'static MemoryMap) -> Self {
+        BootInfoFrameAllocator {
+            memory_map,
+            next: 0,
+        }
+    }
+
+    /// Returns an iterator over the usable frames specified in the memory map.
+    fn usable_frames(&self) -> impl Iterator<Item = PhysFrame> {
+        // get usable regions from memory map
+        let regions = self.memory_map.iter();
+        let usable_regions = regions
+            .filter(|r| r.region_type == MemoryRegionType::Usable);
+        // map each region to its address range
+        let addr_ranges = usable_regions
+            .map(|r| r.range.start_addr()..r.range.end_addr());
+        // transform to an iterator of frame start addresses
+        let frame_addresses = addr_ranges.flat_map(|r| r.step_by(4096));
+        // create `PhysFrame` types from the start addresses
+        frame_addresses.map(|addr| PhysFrame::containing_address(PhysAddr::new(addr)))
+    }
+}
+
+unsafe impl FrameAllocator<Size4KiB> for BootInfoFrameAllocator {
+    fn allocate_frame(&mut self) -> Option<PhysFrame> {
+        let frame = self.usable_frames().nth(self.next);
+        self.next += 1;
+        frame
+    }
+}