Add an AreaFrameAllocator

posts/DRAFT-allocating-frames.md

@@ -200,24 +200,87 @@ impl Frame {
 }
 ```
 
-The allocator struct looks like this:
+### The Allocator
+Now we can put everything together and create the frame allocator. It looks like this:
 
 ```rust
-struct AreaFrameAllocator {
-    first_used_frame: Frame,
-    last_used_frame: Frame,
-    current_area: Option<MemoryArea>,
+use memory::Frame;
+use multiboot2::{MemoryAreaIter, MemoryArea};
+
+pub struct AreaFrameAllocator {
+    next_free_frame: Frame,
+    current_area: Option<&'static MemoryArea>,
     areas: MemoryAreaIter,
+    kernel_start: Frame,
+    kernel_end: Frame,
+    multiboot_start: Frame,
+    multiboot_end: Frame,
 }
 ```
-TODO
+The `next_free_frame` field is a simple counter that is increased every time we return a frame. The `current_area` field holds the memory area that contains `next_free_frame`. If `next_free_frame` leaves this area, we will look for the next one in `areas`. The `{kernel, multiboot}_{start, end}` fields are used to avoid returning already used fields.
 
-To allocate a frame we try to find one in the current area and update the first/last used bounds. If we can't find one, we look for the new area with the minimal start address, that still contains free frames. If the current area is `None`, there are no free frames left.
+```rust
+pub fn new(kernel_start: usize, kernel_end: usize,
+      multiboot_start: usize, multiboot_end: usize,
+      memory_areas: MemoryAreaIter) -> AreaFrameAllocator
+{
+    let mut allocator = AreaFrameAllocator {
+        next_free_frame: Frame::containing_address(0),
+        current_area: None,
+        areas: memory_areas,
+        kernel_start: Frame::containing_address(kernel_start),
+        kernel_end: Frame::containing_address(kernel_end),
+        multiboot_start: Frame::containing_address(multiboot_start),
+        multiboot_end: Frame::containing_address(multiboot_end),
+    };
+    allocator.choose_next_area();
+    allocator
+}
+```
 
-TODO
+```rust
+fn choose_next_area(&mut self) {
+    self.current_area = self.areas.clone().filter(|area| {
+        let address = area.base_addr + area.length - 1;
+        Frame::containing_address(address as usize) >= self.next_free_frame
+    }).min_by(|area| area.base_addr);
+}
+```
 
-### Unit Tests
-TODO
+```rust
+pub fn allocate_frame(&mut self) -> Option<Frame> {
+    match self.current_area {
+        None => None,
+        Some(area) => {
+            let frame = self.next_free_frame;
+            let current_area_last_frame = {
+                let address = area.base_addr + area.length - 1;
+                Frame::containing_address(address as usize)
+            };
+
+            if frame > current_area_last_frame {
+                self.choose_next_area()
+            } else if frame >= self.kernel_start &&
+                frame <= self.kernel_end
+            {
+                self.next_free_frame = Frame {
+                  number: self.kernel_end.number + 1
+                }
+            } else if frame >= self.multiboot_start &&
+                frame <= self.multiboot_end
+            {
+                self.next_free_frame = Frame {
+                  number: self.multiboot_end.number + 1
+                }
+            } else {
+                self.next_free_frame.number += 1;
+                return Some(frame);
+            }
+            self.allocate_frame()
+        }
+    }
+}
+```
 
 ## Remapping the Kernel Sections
 We can use the ELF section tag to write a skeleton that remaps the kernel correctly:

src/lib.rs

@@ -13,7 +13,7 @@
 // limitations under the License.
 
 #![feature(no_std, lang_items)]
-#![feature(const_fn, unique, core_str_ext)]
+#![feature(const_fn, unique, core_str_ext, iter_cmp)]
 #![no_std]
 
 extern crate rlibc;
@@ -54,6 +54,9 @@ pub extern fn rust_main(multiboot_information_address: usize) {
     println!("kernel start: 0x{:x}, kernel end: 0x{:x}", kernel_start, kernel_end);
     println!("multiboot start: 0x{:x}, multiboot end: 0x{:x}", multiboot_start, multiboot_end);
 
+    let mut frame_allocator = memory::AreaFrameAllocator::new(kernel_start as usize,
+        kernel_end as usize, multiboot_start, multiboot_end, memory_map_tag.memory_areas());
+
     loop{}
 }
 

src/memory/area_frame_allocator.rs

@@ -0,0 +1,67 @@
+use memory::Frame;
+use multiboot2::{MemoryAreaIter, MemoryArea};
+
+/// A frame allocator that uses the memory areas from the multiboot information structure as
+/// source. The {kernel, multiboot}_{start, end} fields are used to avoid returning memory that is
+/// already in use.
+///
+/// `kernel_end` and `multiboot_end` are _inclusive_ bounds.
+pub struct AreaFrameAllocator {
+    next_free_frame: Frame,
+    current_area: Option<&'static MemoryArea>,
+    areas: MemoryAreaIter,
+    kernel_start: Frame,
+    kernel_end: Frame,
+    multiboot_start: Frame,
+    multiboot_end: Frame,
+}
+
+impl AreaFrameAllocator {
+    pub fn new(kernel_start: usize, kernel_end: usize, multiboot_start: usize, multiboot_end: usize,
+        memory_areas: MemoryAreaIter) -> AreaFrameAllocator
+    {
+        let mut allocator = AreaFrameAllocator {
+            next_free_frame: Frame::containing_address(0),
+            current_area: None,
+            areas: memory_areas,
+            kernel_start: Frame::containing_address(kernel_start),
+            kernel_end: Frame::containing_address(kernel_end),
+            multiboot_start: Frame::containing_address(multiboot_start),
+            multiboot_end: Frame::containing_address(multiboot_end),
+        };
+        allocator.choose_next_area();
+        allocator
+    }
+
+    pub fn allocate_frame(&mut self) -> Option<Frame> {
+        match self.current_area {
+            None => None,
+            Some(area) => {
+                let frame = self.next_free_frame;
+                let current_area_last_frame = {
+                    let address = area.base_addr + area.length - 1;
+                    Frame::containing_address(address as usize)
+                };
+
+                if frame > current_area_last_frame {
+                    self.choose_next_area()
+                } else if frame >= self.kernel_start && frame <= self.kernel_end {
+                    self.next_free_frame = Frame{ number: self.kernel_end.number + 1 }
+                } else if frame >= self.multiboot_start && frame <= self.multiboot_end {
+                    self.next_free_frame = Frame{ number: self.multiboot_end.number + 1 }
+                } else {
+                    self.next_free_frame.number += 1;
+                    return Some(frame);
+                }
+                self.allocate_frame()
+            }
+        }
+    }
+
+    fn choose_next_area(&mut self) {
+        self.current_area = self.areas.clone().filter(|area| {
+            let address = area.base_addr + area.length - 1;
+            Frame::containing_address(address as usize) >= self.next_free_frame
+        }).min_by(|area| area.base_addr);
+    }
+}

src/memory/mod.rs

@@ -1,3 +1,7 @@
+pub use self::area_frame_allocator::AreaFrameAllocator;
+
+mod area_frame_allocator;
+
 pub const PAGE_SIZE: usize = 4096;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]