add a class to allocate and manage user space stacks

zion/memory/user_stack_manager.h

@@ -0,0 +1,30 @@
+#pragma once
+
+#include <stdint.h>
+
+// Per-process class to manage user stacks.
+//
+// User stacks live at
+// 0x00007FF0 00000000 - 0x00007FFF FFFFFFFF
+// and grow downwards.
+//
+// Each user stack gets 1 MiB (0x100000 bytes)
+// less a page boundary.
+// with 1 page allocated at a time.
+// TODO: consider increasing this.
+class UserStackManager {
+ public:
+  UserStackManager() {}
+  UserStackManager(const UserStackManager&) = delete;
+
+  uint64_t NewUserStack();
+  void FreeUserStack(uint64_t stack_ptr);
+
+ private:
+  const uint64_t kStackMax = 0x00008000'00000000;
+  const uint64_t kStackMin = 0x00007FF0'00000000;
+  const uint64_t kStackSize = 0x100000;
+
+  uint64_t next_stack_ = kStackMax;
+  uint64_t freed_stacks_ = 0;
+};