os: rework kernel architecture

Rework the code structure and architecture of the kernel by separating
platform-dependent and platform-independent code more cleanly. As of
this patchset, full feature parity has not been achieved. Nonetheless, a
sufficient subset of functionality has been ported to the new
architecture to demonstrate the feasibility of the new structure.

1 files changed, 91 insertions, 0 deletions

diff --git a/kernel/src/kapi/memory.cpp b/kernel/src/kapi/memory.cpp
new file mode 100644
index 0000000..d6c84e1
--- /dev/null
+++ b/kernel/src/kapi/memory.cpp
@@ -0,0 +1,91 @@
+#include "kapi/memory.hpp"
+
+#include "kapi/system.hpp"
+
+#include <cstddef>
+#include <optional>
+#include <utility>
+
+namespace teachos::memory
+{
+
+  namespace
+  {
+    struct bad_frame_allocator final : public frame_allocator
+    {
+      bad_frame_allocator static instance;
+
+      auto allocate() noexcept -> std::optional<frame> override
+      {
+        system::panic("Tried to allocate a frame without an active allocator.");
+      }
+
+      auto release(frame) -> void override
+      {
+        system::panic("Tried to release a frame without an active allocator.");
+      }
+    };
+
+    struct bad_page_mapper final : public page_mapper
+    {
+      bad_page_mapper static instance;
+
+      auto map(page, frame, flags) -> std::byte * override
+      {
+        system::panic("Tried to map a page without an active mapper.");
+      }
+
+      auto unmap(page) -> void override
+      {
+        system::panic("Tried to unmap a page without an active mapper.");
+      }
+
+      auto try_unmap(page) noexcept -> bool override
+      {
+        return false;
+      }
+    };
+
+    constinit bad_frame_allocator bad_frame_allocator::instance{};
+    constinit bad_page_mapper bad_page_mapper::instance{};
+  }  // namespace
+
+  // NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
+  constinit auto active_frame_allocator = static_cast<frame_allocator *>(&bad_frame_allocator::instance);
+  // NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
+  constinit auto active_page_mapper = static_cast<page_mapper *>(&bad_page_mapper::instance);
+
+  auto set_frame_allocator(frame_allocator & allocator) -> std::optional<frame_allocator *>
+  {
+    if (&allocator == active_frame_allocator)
+    {
+      return {};
+    }
+    return std::exchange(active_frame_allocator, &allocator);
+  }
+
+  auto set_page_mapper(page_mapper & mapper) -> std::optional<page_mapper *>
+  {
+    if (&mapper == active_page_mapper)
+    {
+      return {};
+    }
+    return std::exchange(active_page_mapper, &mapper);
+  }
+
+  auto allocate_frame() -> std::optional<frame>
+  {
+    return active_frame_allocator->allocate();
+  }
+
+  auto map(page page, frame frame) -> std::byte *
+  {
+    return active_page_mapper->map(page, frame, page_mapper::flags::empty);
+  }
+
+  auto unmap(page page) -> void
+  {
+    return active_page_mapper->unmap(page);
+  }
+
+}  // namespace teachos::memory
\ No newline at end of file