From 769539698050d9c14064c3830d728f5fc7226a5e Mon Sep 17 00:00:00 2001
From: Drew Galbraith <dgalbraith33@gmail.com>
Date: Fri, 24 Nov 2023 16:16:25 -0800
Subject: [PATCH] [Zion] Add released User Stacks to pool for reuse.

---
 zion/memory/constants.h            |  5 +++++
 zion/memory/user_stack_manager.cpp | 15 +++++++++------
 zion/memory/user_stack_manager.h   | 10 +++++-----
 3 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/zion/memory/constants.h b/zion/memory/constants.h
index 335746f..2c17ba3 100644
--- a/zion/memory/constants.h
+++ b/zion/memory/constants.h
@@ -3,6 +3,7 @@
 #include <stdint.h>
 
 const uint64_t KiB = 0x400;
+const uint64_t MiB = KiB * KiB;
 const uint64_t kPageSize = 4 * KiB;
 
 const uint64_t kKernelSlabHeapStart = 0xFFFF'FFFF'4000'0000;
@@ -20,3 +21,7 @@ const uint64_t kKernelStackEnd = 0xFFFF'FFFF'A000'0000;
 
 const uint64_t kKernelStackSize = 3 * kPageSize;
 const uint64_t kKernelStackOffset = 4 * kPageSize;
+
+const uint64_t kUserStackMin = 0x00007FF0'00000000;
+const uint64_t kUserStackMax = 0x00008000'00000000;
+const uint64_t kUserStackSize = MiB;
diff --git a/zion/memory/user_stack_manager.cpp b/zion/memory/user_stack_manager.cpp
index d714b8a..ffb91ae 100644
--- a/zion/memory/user_stack_manager.cpp
+++ b/zion/memory/user_stack_manager.cpp
@@ -4,12 +4,16 @@
 #include "memory/paging_util.h"
 
 uint64_t UserStackManager::NewUserStack() {
+  if (!freed_stacks_.empty()) {
+    return freed_stacks_.PopFront();
+  }
+
   uint64_t stack = next_stack_;
-  next_stack_ -= kStackSize;
-  if (stack <= kStackMin) {
+  next_stack_ -= kUserStackSize;
+  if (stack <= kUserStackMin) {
     panic("Out of user stacks!");
   }
-  if (stack == kStackMax) {
+  if (stack == kUserStackMax) {
     // Add a additional page boudary between kernel and user space.
     stack -= 0x1000;
   }
@@ -18,12 +22,11 @@ uint64_t UserStackManager::NewUserStack() {
 }
 
 void UserStackManager::FreeUserStack(uint64_t stack_ptr) {
-  freed_stacks_++;
-  dbgln("{} freed user stacks", freed_stacks_);
+  freed_stacks_.PushBack(stack_ptr);
 }
 
 bool UserStackManager::IsValidStack(uint64_t vaddr) {
-  if (vaddr < next_stack_ || vaddr > (kStackMax - 0x1000)) {
+  if (vaddr < next_stack_ || vaddr > (kUserStackMax - 0x1000)) {
     return false;
   }
   // Checks if the address is in the first page of the stack.
diff --git a/zion/memory/user_stack_manager.h b/zion/memory/user_stack_manager.h
index 5ed7a0f..fa549c4 100644
--- a/zion/memory/user_stack_manager.h
+++ b/zion/memory/user_stack_manager.h
@@ -1,7 +1,10 @@
 #pragma once
 
+#include <glacier/container/linked_list.h>
 #include <stdint.h>
 
+#include "memory/constants.h"
+
 // Per-process class to manage user stacks.
 //
 // User stacks live at
@@ -24,10 +27,7 @@ class UserStackManager {
   bool IsValidStack(uint64_t vaddr);
 
  private:
-  const uint64_t kStackMax = 0x00008000'00000000;
-  const uint64_t kStackMin = 0x00007FF0'00000000;
-  const uint64_t kStackSize = 0x100000;
+  uint64_t next_stack_ = kUserStackMax;
 
-  uint64_t next_stack_ = kStackMax;
-  uint64_t freed_stacks_ = 0;
+  glcr::LinkedList<uint64_t> freed_stacks_;
 };