From c2f48fb5df0981df1df23de2b277274f9fe75080 Mon Sep 17 00:00:00 2001
From: Benji Dial <benji@benjidial.net>
Date: Mon, 8 Jan 2024 22:28:41 -0500
Subject: first commit

---
 kernel/paging.cpp | 115 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 115 insertions(+)
 create mode 100644 kernel/paging.cpp

(limited to 'kernel/paging.cpp')

diff --git a/kernel/paging.cpp b/kernel/paging.cpp
new file mode 100644
index 0000000..3bd27d0
--- /dev/null
+++ b/kernel/paging.cpp
@@ -0,0 +1,115 @@
+#include <mercury/kernel/utility.hpp>
+#include <mercury/kernel/paging.hpp>
+
+//see also ../documentation/memory.txt
+
+extern "C" {
+  uint64_t __kernel_p4_paddr;
+}
+
+namespace mercury::kernel::paging {
+
+  static constexpr uint64_t kernel_vram_start = 0xffffffffc0000000;
+  static constexpr uint64_t kernel_vram_pages = 261888;
+  static constexpr uint64_t kernel_stack_bottom = 0xfffffffffff01000;
+  static constexpr uint64_t kernel_stack_top = 0xfffffffffffff000;
+
+  static constexpr uint64_t pram_pages = 1 << 23;
+  static uint64_t pram_usage_bitmap[pram_pages / 64];
+
+  void mark_all_pram_used() {
+    utility::mark_bitmap_region_one(pram_usage_bitmap, 0, pram_pages);
+  }
+
+  void mark_pram_region_free(uint64_t start_addr, uint64_t end_addr) {
+    utility::mark_bitmap_region_zero(
+      pram_usage_bitmap, start_addr / 4096,
+      utility::min(end_addr / 4096, pram_pages));
+  }
+
+  [[gnu::aligned(4096)]] uint64_t kernel_p4[512];
+  [[gnu::aligned(4096)]] uint64_t kernel_p3[512];
+  [[gnu::aligned(4096)]] uint64_t kernel_p2[512];
+  [[gnu::aligned(4096)]] uint64_t kernel_p1s[512 * 512];
+
+  static uint64_t encode_pte(
+      uint64_t addr, bool user, bool write, bool execute) {
+    return (addr & 0x0000ffffffffffff) | (execute ? 0 : (1ULL << 63))
+         | (user << 2) | (write << 1) | 1;
+  }
+
+  void init_kernel_page_tables(uint64_t kernel_offset) {
+    __kernel_p4_paddr = (uint64_t)kernel_p4 - kernel_offset;
+    for (int i = 0; i < 511; ++i)
+      kernel_p4[i] = 0;
+    kernel_p4[511] = encode_pte(
+      (uint64_t)kernel_p3 - kernel_offset, false, true, true);
+    for (int i = 0; i < 511; ++i)
+      kernel_p3[i] = 0;
+    kernel_p3[511] = encode_pte(
+      (uint64_t)kernel_p2 - kernel_offset, false, true, true);
+    for (int i = 0; i < 512; ++i)
+      kernel_p2[i] = encode_pte(
+        (uint64_t)kernel_p1s + 4096 * i - kernel_offset, false, true, true);
+    for (int i = 0; i < 512 * 512; ++i)
+      kernel_p1s[i] = 0;
+  }
+
+  void map_kernel_page(
+      uint64_t paddr, uint64_t vaddr, bool write, bool execute) {
+    uint64_t i = (vaddr - kernel_vram_start) / 4096;
+    kernel_p1s[i] = encode_pte(paddr, false, write, execute);
+  }
+
+  static uint64_t take_pram_page() {
+    for (uint64_t i = 0; i < pram_pages / 64; ++i)
+      if (pram_usage_bitmap[i] != 0xffffffffffffffff)
+        for (int j = 0; j < 64; ++j)
+          if (!(pram_usage_bitmap[i] & (1ULL << j))) {
+            pram_usage_bitmap[i] |= (1ULL << j);
+            return 4096 * (i * 64 + j);
+          }
+    //TODO: handle error
+    return 0;
+  }
+
+  void map_kernel_stack() {
+    for (uint64_t vaddr = kernel_stack_bottom;
+         vaddr < kernel_stack_top; vaddr += 4096)
+      map_kernel_page(take_pram_page(), vaddr, true, false);
+  }
+
+  uint64_t find_unmapped_vram_region(uint64_t page_count) {
+    uint64_t start = 0;
+    uint64_t len = 0;
+    for (uint64_t i = 0; i < kernel_vram_pages; ++i)
+      if (kernel_p1s[i] == 0) {
+        ++len;
+        if (len == page_count)
+          return start * 4096 + kernel_vram_start;
+      }
+      else {
+        start = i + 1;
+        len = 0;
+      }
+    //TODO: handle error
+    return 0;
+  }
+
+  uint64_t get_used_vram_page_count() {
+    uint64_t count = 0;
+    for (uint64_t i = 0; i < kernel_vram_pages; ++i)
+      if (kernel_p1s[i] != 0)
+        ++count;
+    return count;
+  }
+
+  uint64_t get_free_pram_page_count() {
+    uint64_t used_count = 0;
+    for (uint64_t i = 0; i < pram_pages / 64; ++i)
+      for (uint64_t j = 0; j < 64; ++j)
+        used_count += (pram_usage_bitmap[i] >> j) & 1;
+    return pram_pages - used_count;
+  }
+
+}
-- 
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