From c6ae501bc6e7b15c69f11f175e61a482391723a5 Mon Sep 17 00:00:00 2001
From: Erin <erin@hashbang.sh>
Date: Tue, 31 Oct 2017 22:10:54 -0500
Subject: [PATCH] x86_64::memory: implement an allocator for stacks

---
 src/arch/x86_64/memory/mod.rs             |  2 +
 src/arch/x86_64/memory/stack_allocator.rs | 79 +++++++++++++++++++++++++++++++
 2 files changed, 81 insertions(+)
 create mode 100644 src/arch/x86_64/memory/stack_allocator.rs

diff --git a/src/arch/x86_64/memory/mod.rs b/src/arch/x86_64/memory/mod.rs
index 25e6a96..d761900 100644
--- a/src/arch/x86_64/memory/mod.rs
+++ b/src/arch/x86_64/memory/mod.rs
@@ -3,6 +3,7 @@
 //! Heavly inspired/lovingly ripped off from Phil Oppermann's [os.phil-opp.com](http://os.phil-opp.com/).
 
 mod area_frame_allocator;
+mod stack_allocator;
 mod paging;
 
 use multiboot2::BootInformation;
@@ -11,6 +12,7 @@ use ::alloca;
 
 pub use self::area_frame_allocator::AreaFrameAllocator;
 use self::paging::Page;
+pub use self::stack_allocator::{Stack, StackAllocator};
 
 /// The physical size of each frame.
 pub const PAGE_SIZE: usize = 4096;
diff --git a/src/arch/x86_64/memory/stack_allocator.rs b/src/arch/x86_64/memory/stack_allocator.rs
new file mode 100644
index 0000000..0ccdcb1
--- /dev/null
+++ b/src/arch/x86_64/memory/stack_allocator.rs
@@ -0,0 +1,79 @@
+use memory::paging::{self, Page, PageIter, ActivePageTable};
+use memory::{PAGE_SIZE, FrameAllocator};
+
+#[derive(Debug)]
+pub struct Stack {
+    top: usize,
+    bottom: usize,
+}
+
+impl Stack {
+    fn new(top: usize, bottom: usize) -> Stack {
+        assert!(top > bottom,
+            "Stack top must be higher in memory than the bottom");
+
+        Stack {
+            top: top,
+            bottom: bottom,
+        }
+    }
+
+    pub fn top(&self) -> usize    { self.top }
+    pub fn bottom(&self) -> usize { self.bottom }
+}
+
+
+pub struct StackAllocator {
+    range: PageIter,
+}
+
+impl StackAllocator {
+    pub fn new(page_range: PageIter) -> StackAllocator {
+        StackAllocator { range: page_range }
+    }
+
+    /// Allocate a stack.
+    ///
+    /// Note: `size` is given in pages.
+    pub fn alloc_stack<A>(&mut self,
+                 active_table: &mut ActivePageTable,
+                 frame_alloc: &mut A,
+                 size: usize) -> Option<Stack>
+        where A: FrameAllocator
+    {
+        // zero-size stacks are nonsensical
+        if size == 0 {
+            return None;
+        }
+
+        let mut range = self.range.clone();
+
+        // try to alloc stack, guard pages
+        let guard_page = range.next();
+        let stack_start = range.next();
+        let stack_end = if size == 1 {
+            stack_start
+        } else {
+            range.nth(size - 2)
+        };
+
+        match (guard_page, stack_start, stack_end) {
+            (Some(_), Some(start), Some(end)) => {
+                // writeback
+                self.range = range;
+
+                // map stack pages -> physical frames
+                for page in Page::range_inclusive(start, end) {
+                    active_table.map(page, paging::WRITABLE, frame_alloc);
+                }
+
+                // create a new stack
+
+                let top = end.start_address() + PAGE_SIZE;
+
+                Some(Stack::new(top, start.start_address()))
+            }
+            _ => None, // whoops not enough frames
+        }
+    }
+}