paging: Use dedicated paging::Manager

Like PortIO the page tables are a fundamentally global structure. By
moving the paging logic to a separate file, the requirement for
exclusive access is now correctly modeled with Rust types.

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

layout.ld

@@ -35,7 +35,7 @@ SECTIONS
 	/* Memory for identity mapping, keep synced with ADDRESS_SPACE_GIB */
 	address_space_gib = 4;
 	. = ALIGN(4K);
-	pml2t = .;
+	pml2ts = .;
 	. += 4K * address_space_gib;
 
 	ASSERT((. <= ram_max - stack_size), "firmware size too big for RAM region")

src/main.rs

@@ -37,6 +37,7 @@ mod efi;
 mod fat;
 mod loader;
 mod mem;
+mod paging;
 mod part;
 mod pci;
 mod pe;
@@ -60,32 +61,6 @@ fn panic(_: &PanicInfo) -> ! {
     loop {}
 }
 
-/// Setup page tables to provide an identity mapping over the full 4GiB range
-fn setup_pagetables() {
-    type PageTable = [u64; 512];
-
-    extern "C" {
-        static pml3t: PageTable;
-        static pml2t: PageTable;
-        static address_space_gib: u8;
-    }
-    let num_gib = unsafe { &address_space_gib } as *const _ as usize as u64;
-    log!("Setting up {} GiB identity mapping", num_gib);
-
-    let pml2t_addr = unsafe { pml2t.as_ptr() } as usize as u64;
-    let pte = mem::MemoryRegion::new(pml2t_addr, num_gib * 4096);
-    for i in 0..(512 * num_gib) {
-        pte.io_write_u64(i * 8, (i << 21) + 0x83u64)
-    }
-
-    let pde = mem::MemoryRegion::from_slice(unsafe { &pml3t });
-    for i in 0..num_gib {
-        pde.io_write_u64(i * 8, (pml2t_addr + (0x1000u64 * i)) | 0x03);
-    }
-
-    log!("Page tables setup");
-}
-
 // Enable SSE2 for XMM registers (needed for EFI calling)
 fn enable_sse() {
     let mut cr0 = Cr0::read();
@@ -182,7 +157,7 @@ fn boot_from_device(device: &mut block::VirtioBlockDevice) -> bool {
 pub extern "C" fn rust64_start() -> ! {
     log!("\nStarting..");
     enable_sse();
-    setup_pagetables();
+    paging::MANAGER.borrow_mut().setup();
 
     pci::print_bus();
 

src/paging.rs

@@ -0,0 +1,68 @@
+use atomic_refcell::AtomicRefCell;
+use x86_64::{
+    registers::control::Cr3,
+    structures::paging::{PageSize, PageTable, PageTableFlags, PhysFrame, Size2MiB},
+    PhysAddr,
+};
+
+// Keep in sync with address_space_gib in layout.ld
+const ADDRESS_SPACE_GIB: usize = 4;
+
+pub static MANAGER: AtomicRefCell<Manager> = AtomicRefCell::new(Manager);
+pub struct Manager;
+
+extern "C" {
+    static mut pml4t: PageTable;
+    static mut pml3t: PageTable;
+    static mut pml2ts: [PageTable; ADDRESS_SPACE_GIB];
+}
+
+struct Tables<'a> {
+    l4: &'a mut PageTable,
+    l3: &'a mut PageTable,
+    l2s: &'a mut [PageTable],
+}
+
+impl Manager {
+    fn tables(&mut self) -> Tables {
+        Tables {
+            l4: unsafe { &mut pml4t },
+            l3: unsafe { &mut pml3t },
+            l2s: unsafe { &mut pml2ts },
+        }
+    }
+
+    pub fn setup(&mut self) {
+        log!("Setting up {} GiB identity mapping", ADDRESS_SPACE_GIB);
+        let Tables { l4, l3, l2s } = self.tables();
+
+        let pt_flags = PageTableFlags::PRESENT | PageTableFlags::WRITABLE;
+        // Setup Identity map using L2 huge pages
+        let mut next_addr = PhysAddr::new(0);
+        for l2 in l2s.iter_mut() {
+            for l2e in l2.iter_mut() {
+                l2e.set_addr(next_addr, pt_flags | PageTableFlags::HUGE_PAGE);
+                next_addr += Size2MiB::SIZE;
+            }
+        }
+
+        // Point L3 at L2s
+        for (i, l2) in l2s.iter().enumerate() {
+            l3[i].set_addr(phys_addr(l2), pt_flags);
+        }
+
+        // Point L4 at L3
+        l4[0].set_addr(phys_addr(l3), pt_flags);
+
+        // Point Cr3 at PML4
+        let cr3_flags = Cr3::read().1;
+        let pml4t_frame = PhysFrame::from_start_address(phys_addr(l4)).unwrap();
+        unsafe { Cr3::write(pml4t_frame, cr3_flags) };
+        log!("Page tables setup");
+    }
+}
+
+// Map a virtual address to a PhysAddr (assumes identity mapping)
+fn phys_addr<T>(virt_addr: *const T) -> PhysAddr {
+    PhysAddr::new(virt_addr as u64)
+}