add a class to allocate and manage user space stacks

zion/memory/user_stack_manager.cpp

@@ -0,0 +1,23 @@
+#include "memory/user_stack_manager.h"
+
+#include "debug/debug.h"
+#include "memory/paging_util.h"
+
+uint64_t UserStackManager::NewUserStack() {
+  uint64_t stack = next_stack_;
+  next_stack_ -= kStackSize;
+  if (stack <= kStackMin) {
+    panic("Out of user stacks!");
+  }
+  if (stack == kStackMax) {
+    // Add a additional page boudary between kernel and user space.
+    stack -= 0x1000;
+  }
+  EnsureResident(stack - 1, 1);
+  return stack;
+}
+
+void UserStackManager::FreeUserStack(uint64_t stack_ptr) {
+  freed_stacks_++;
+  dbgln("%u freed user stacks", freed_stacks_);
+}