paging: Put tables in static mut

We don't actually need an AtomicRefcell here. It's fine for
paging::setup() to run multiple times, it's idempontent and we only
do writes.

We also place the page tables in #[no_mangle] static mut variables so
they can be linked from assembly code.

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

Author: josephlr

src/main.rs

@@ -156,7 +156,7 @@ fn boot_from_device(device: &mut block::VirtioBlockDevice) -> bool {
 pub extern "C" fn rust64_start() -> ! {
     log!("\nStarting..");
     enable_sse();
-    paging::MANAGER.borrow_mut().setup();
+    paging::setup();
 
     pci::print_bus();
 

src/paging.rs

@@ -1,56 +1,51 @@
-use atomic_refcell::AtomicRefCell;
 use x86_64::{
     registers::control::Cr3,
     structures::paging::{PageSize, PageTable, PageTableFlags, PhysFrame, Size2MiB},
     PhysAddr,
 };
 
-// This is the number of GiB we will identity map.
+// Amount of memory we identity map in setup(), max 512 GiB.
 const ADDRESS_SPACE_GIB: usize = 4;
 
-pub static MANAGER: AtomicRefCell<Manager> = AtomicRefCell::new(Manager::new());
-pub struct Manager {
-    l4: PageTable,
-    l3: PageTable,
-    l2s: [PageTable; ADDRESS_SPACE_GIB],
-}
-
-impl Manager {
-    const fn new() -> Self {
-        Manager {
-            l4: PageTable::new(),
-            l3: PageTable::new(),
-            l2s: [PageTable::new(); ADDRESS_SPACE_GIB],
+// Put the Page Tables in static muts to make linking easier
+#[no_mangle]
+static mut L4_TABLE: PageTable = PageTable::new();
+#[no_mangle]
+static mut L3_TABLE: PageTable = PageTable::new();
+#[no_mangle]
+static mut L2_TABLES: [PageTable; ADDRESS_SPACE_GIB] = [PageTable::new(); ADDRESS_SPACE_GIB];
+
+pub fn setup() {
+    // SAFETY: This function is idempontent and only writes to static memory and
+    // CR3. Thus, it is safe to run multiple times or on multiple threads.
+    let (l4, l3, l2s) = unsafe { (&mut L4_TABLE, &mut L3_TABLE, &mut L2_TABLES) };
+    log!("Setting up {} GiB identity mapping", ADDRESS_SPACE_GIB);
+    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;
         }
     }
 
-    pub fn setup(&mut self) {
-        log!("Setting up {} GiB identity mapping", ADDRESS_SPACE_GIB);
-
-        let pt_flags = PageTableFlags::PRESENT | PageTableFlags::WRITABLE;
-        // Setup Identity map using L2 huge pages
-        let mut next_addr = PhysAddr::new(0);
-        for l2 in self.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 self.l2s.iter().enumerate() {
-            self.l3[i].set_addr(phys_addr(l2), pt_flags);
-        }
+    // Point L3 at L2s
+    for (i, l2) in l2s.iter().enumerate() {
+        l3[i].set_addr(phys_addr(l2), pt_flags);
+    }
 
-        // Point L4 at L3
-        self.l4[0].set_addr(phys_addr(&self.l3), 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(&self.l4)).unwrap();
-        unsafe { Cr3::write(pml4t_frame, cr3_flags) };
-        log!("Page tables setup");
+    // Point Cr3 at L4
+    let (cr3_frame, cr3_flags) = Cr3::read();
+    let l4_frame = PhysFrame::from_start_address(phys_addr(l4)).unwrap();
+    if cr3_frame != l4_frame {
+        unsafe { Cr3::write(l4_frame, cr3_flags) };
     }
+    log!("Page tables setup");
 }
 
 // Map a virtual address to a PhysAddr (assumes identity mapping)