limine: Do not drop unconditional mapping of first 4GiB

PROTOCOL.md

@@ -98,6 +98,9 @@ below.
 Alongside the loaded kernel, the bootloader will set up memory mappings such
 that every usable, bootloader reclaimable, framebuffer, or kernel/modules
 memory map region is mapped at HHDM offset + its physical address.
+Additionally, the whole 0->4GiB physical memory region will also be mapped
+at HHDM offset + physical address, regardless of the contents of the
+memory map.
 These mappings are supervisor, read, write, execute (-rwx), and they use
 WB caching (on x86_64).
 

common/protos/limine.c

@@ -69,12 +69,18 @@ static pagemap_t build_pagemap(int paging_mode, bool nx, struct elf_range *range
     for (size_t i = 0; i < _memmap_entries; i++)
         _memmap[i] = memmap[i];
 
+    // Map 0->4GiB range to HHDM
+    for (uint64_t i = 0; i < 0x100000000; i += 0x40000000) {
+        map_page(pagemap, direct_map_offset + i, i, VMM_FLAG_WRITE, Size1GiB);
+    }
+
+    print_memmap(memmap, memmap_entries);
+
     // Map all free memory regions to the higher half direct map offset
     for (size_t i = 0; i < _memmap_entries; i++) {
         if (_memmap[i].type != MEMMAP_USABLE
          && _memmap[i].type != MEMMAP_BOOTLOADER_RECLAIMABLE
-         && _memmap[i].type != MEMMAP_KERNEL_AND_MODULES
+         && _memmap[i].type != MEMMAP_KERNEL_AND_MODULES) {
-         && _memmap[i].type != MEMMAP_FRAMEBUFFER) {
             continue;
         }
 
@@ -82,24 +88,25 @@ static pagemap_t build_pagemap(int paging_mode, bool nx, struct elf_range *range
         uint64_t length = _memmap[i].length;
         uint64_t top    = base + length;
 
+        if (base < 0x100000000) {
+            base = 0x100000000;
+        }
+
+        if (base >= top) {
+            continue;
+        }
+
         uint64_t aligned_base   = ALIGN_DOWN(base, 0x40000000);
         uint64_t aligned_top    = ALIGN_UP(top, 0x40000000);
         uint64_t aligned_length = aligned_top - aligned_base;
 
         for (uint64_t j = 0; j < aligned_length; j += 0x40000000) {
             uint64_t page = aligned_base + j;
-#if defined (__x86_64__) || defined (__i386__)
-            // XXX we do this as a quick and dirty way to switch to the higher half
-            if (_memmap[i].type == MEMMAP_BOOTLOADER_RECLAIMABLE) {
-                map_page(pagemap, page, page, VMM_FLAG_WRITE, Size1GiB);
-            }
-#endif
             map_page(pagemap, direct_map_offset + page, page, VMM_FLAG_WRITE, Size1GiB);
         }
     }
 
-    // Map the framebuffer as uncacheable
+    // Map the framebuffer with appropriate permissions
-#if defined (__aarch64__)
     for (size_t i = 0; i < _memmap_entries; i++) {
         if (_memmap[i].type != MEMMAP_FRAMEBUFFER) {
             continue;
@@ -118,6 +125,12 @@ static pagemap_t build_pagemap(int paging_mode, bool nx, struct elf_range *range
             map_page(pagemap, direct_map_offset + page, page, VMM_FLAG_WRITE | VMM_FLAG_FB, Size4KiB);
         }
     }
+
+    // XXX we do this as a quick and dirty way to switch to the higher half
+#if defined (__x86_64__) || defined (__i386__)
+    for (uint64_t i = 0; i < 0x100000000; i += 0x40000000) {
+        map_page(pagemap, i, i, VMM_FLAG_WRITE, Size1GiB);
+    }
 #endif
 
     return pagemap;