From 0f847cdb204f09bf6a31be4bd87c23d2fd14e527 Mon Sep 17 00:00:00 2001
From: Erin <erin@hashbang.sh>
Date: Sun, 4 Jun 2017 17:48:43 -0500
Subject: [PATCH] Implement a TemporaryPage

---
 src/arch/x86_64/memory/paging/temporary_page.rs | 75 +++++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100644 src/arch/x86_64/memory/paging/temporary_page.rs

diff --git a/src/arch/x86_64/memory/paging/temporary_page.rs b/src/arch/x86_64/memory/paging/temporary_page.rs
new file mode 100644
index 0000000..6a96a29
--- /dev/null
+++ b/src/arch/x86_64/memory/paging/temporary_page.rs
@@ -0,0 +1,75 @@
+use arch::x86_64::memory::{Frame, FrameAllocator};
+use super::{Page, Table, ActivePageTable};
+use super::table::{Level1, Level4};
+use super::{VirtualAddress};
+
+pub struct TemporaryPage {
+    page: Page,
+    allocator: TinyAllocator,
+}
+
+impl TemporaryPage {
+    pub fn new<A>(page: Page, allocator: &mut A) -> TemporaryPage
+            where A: FrameAllocator {
+        TemporaryPage {
+            page: page,
+            allocator: TinyAllocator::new(allocator),
+        }
+    }
+
+    /// Maps the temporary page to the given frame, using the active table.
+    /// Returns the start address of the temporary page in VRAM.
+    pub fn map(&mut self, frame: Frame, active_table: &mut ActivePageTable) -> VirtualAddress {
+        use super::entry::WRITABLE;
+
+        assert!(active_table.page_to_frame(self.page).is_none(),
+            "Temporary page is already mapped!");
+        active_table.map_to(self.page, frame, WRITABLE, &mut self.allocator);
+
+        self.page.start_address()
+    }
+
+    /// Maps the temporary page to the given page table frame, using the active table.
+    /// Returns a &mut reference to the now-mapped table.
+    pub fn map_table_frame(&mut self, frame: Frame, active_table: &mut ActivePageTable)
+            -> &mut Table<Level1> {
+        unsafe { &mut *(self.map(frame, active_table) as *mut Table<Level1>) }
+    }
+
+    pub fn unmap(&mut self, active_table: &mut ActivePageTable) {
+        active_table.unmap(self.page, &mut self.allocator);
+    }
+}
+
+/// A tiny frame allocator which can only hold three frames,
+/// used to temporarily create a P3, P2, and P1 table.
+struct TinyAllocator([Option<Frame>; 3]);
+impl TinyAllocator {
+    fn new<A>(allocator: &mut A) -> TinyAllocator
+            where A: FrameAllocator{
+        let mut f = || allocator.alloc_frame();
+        let frames = [f(), f(), f()];
+        TinyAllocator(frames)
+    }
+}
+impl FrameAllocator for TinyAllocator {
+    fn alloc_frame(&mut self) -> Option<Frame> {
+        for frame in &mut self.0 {
+            if frame.is_some() {
+                return frame.take();
+            }
+        }
+        None
+    }
+
+    fn dealloc_frame(&mut self, frame: Frame) {
+        for stored_frame in &mut self.0 {
+            if stored_frame.is_none() {
+                *stored_frame = Some(frame);
+                return;
+            }
+        }
+
+        panic!("TinyAllocator can only hold 3 frames!");
+    }
+}