kernel/x86_64: Map kernel-side pages as global to avoid TLB flushes

Fixes #34.

kernel/src/arch/MMU.h

@@ -19,6 +19,7 @@ namespace MMU
         NoExecute = 4,
         WriteThrough = 8,
         CacheDisable = 16,
+        Global = 32,
     };
 
     enum class UseHugePages

kernel/src/arch/x86_64/CPU.asm

@@ -25,6 +25,13 @@ enable_nx:
     wrmsr
     ret
 
+global enable_global_pages
+enable_global_pages:
+    mov rax, cr4
+    or ax, 1 << 7
+    mov cr4, rax
+    ret
+
 global load_gdt
 load_gdt:
     cli

kernel/src/arch/x86_64/CPU.cpp

@@ -22,6 +22,7 @@
 
 extern "C" void enable_sse();
 extern "C" void enable_write_protect();
+extern "C" void enable_global_pages();
 extern "C" void enable_nx();
 
 extern void setup_gdt();
@@ -268,6 +269,7 @@ namespace CPU
     void platform_init()
     {
         enable_sse();
+        enable_global_pages();
         // enable_write_protect();
         if (test_nx()) enable_nx();
         else

kernel/src/arch/x86_64/MMU.cpp

@@ -107,6 +107,7 @@ namespace MMU
         if (entry.no_execute) result |= Flags::NoExecute;
         if (entry.write_through) result |= Flags::WriteThrough;
         if (entry.cache_disabled) result |= Flags::CacheDisable;
+        if (entry.global) result |= Flags::Global;
         return result;
     }
 
@@ -181,6 +182,7 @@ namespace MMU
         entry.write_through = has_flag(flags, Flags::WriteThrough);
         entry.cache_disabled = has_flag(flags, Flags::CacheDisable);
         entry.no_execute = has_flag(flags, Flags::NoExecute);
+        entry.global = has_flag(flags, Flags::Global);
         entry.set_address(phys);
     }
 
@@ -249,6 +251,7 @@ namespace MMU
         l1.write_through = has_flag(flags, Flags::WriteThrough);
         l1.cache_disabled = has_flag(flags, Flags::CacheDisable);
         l1.no_execute = has_flag(flags, Flags::NoExecute);
+        l1.global = has_flag(flags, Flags::Global);
         flush_page(virt);
         return {};
     }
@@ -291,7 +294,7 @@ namespace MMU
 
         check(physical_memory_size % ARCH_HUGE_PAGE_SIZE == 0);
         MemoryManager::map_huge_frames_at(physical_memory_base, 0, physical_memory_size / ARCH_HUGE_PAGE_SIZE,
-                                          MMU::ReadWrite | MMU::NoExecute);
+                                          MMU::ReadWrite | MMU::NoExecute | MMU::Global);
 
         g_physical_mapping_base = physical_memory_base;
 

kernel/src/arch/x86_64/MMU.h

@@ -17,7 +17,7 @@ struct [[gnu::packed]] PageTableEntry
             bool accessed : 1;
             bool ignore0 : 1;
             bool larger_pages : 1;
-            bool ignore1 : 1;
+            bool global : 1;
             u8 available : 3;
             u64 address : 48;
             u8 available2 : 3;

kernel/src/memory/MemoryManager.cpp

@@ -44,11 +44,11 @@ namespace MemoryManager
     {
         const usize rodata_size = (usize)(end_of_kernel_rodata - start_of_kernel_rodata);
         const usize rodata_pages = ceil_div(rodata_size, ARCH_PAGE_SIZE);
-        TRY(remap((u64)start_of_kernel_rodata, rodata_pages, MMU::NoExecute));
+        TRY(remap((u64)start_of_kernel_rodata, rodata_pages, MMU::NoExecute | MMU::Global));
 
         const usize data_size = (usize)(end_of_kernel_data - start_of_kernel_data);
         const usize data_pages = ceil_div(data_size, ARCH_PAGE_SIZE);
-        TRY(remap((u64)start_of_kernel_data, data_pages, MMU::NoExecute | MMU::ReadWrite));
+        TRY(remap((u64)start_of_kernel_data, data_pages, MMU::NoExecute | MMU::ReadWrite | MMU::Global));
 
         return {};
     }
@@ -380,7 +380,7 @@ namespace MemoryManager
         while (pages_mapped < count)
         {
             const u64 frame = TRY(alloc_frame());
-            TRY(MMU::map(virt, frame, flags, MMU::UseHugePages::No));
+            TRY(MMU::map(virt, frame, flags | MMU::Global, MMU::UseHugePages::No));
             virt += ARCH_PAGE_SIZE;
             pages_mapped++;
         }
@@ -405,7 +405,7 @@ namespace MemoryManager
 
         while (pages_mapped < count)
         {
-            TRY(MMU::map(virt, phys, flags, MMU::UseHugePages::No));
+            TRY(MMU::map(virt, phys, flags | MMU::Global, MMU::UseHugePages::No));
             virt += ARCH_PAGE_SIZE;
             phys += ARCH_PAGE_SIZE;
             pages_mapped++;