Kernel: Add getter and setter for the X86 CR3 register

This gets rid of a bunch of inline assembly.

Kernel/Arch/i386/CPU.cpp

@@ -178,9 +178,7 @@ static void dump(const RegisterDump& regs)
     u32 cr2;
     asm("movl %%cr2, %%eax"
         : "=a"(cr2));
-    u32 cr3;
+    u32 cr3 = read_cr3();
-    asm("movl %%cr3, %%eax"
-        : "=a"(cr3));
     u32 cr4;
     asm("movl %%cr4, %%eax"
         : "=a"(cr4));
@@ -271,11 +269,8 @@ void page_fault_handler(RegisterDump regs)
     asm("movl %%cr2, %%eax"
         : "=a"(fault_address));
 
-    u32 fault_page_directory;
-    asm("movl %%cr3, %%eax"
-        : "=a"(fault_page_directory));
-
 #ifdef PAGE_FAULT_DEBUG
+    u32 fault_page_directory = read_cr3();
     dbgprintf("%s(%u): ring%u %s page fault in PD=%x, %s%s V%08x\n",
         current ? current->process().name().characters() : "(none)",
         current ? current->pid() : 0,
@@ -716,3 +711,17 @@ void cpu_setup()
         kprintf("x86: No RDRAND support detected. Randomness will be shitty\n");
     }
 }
+
+u32 read_cr3()
+{
+    u32 cr3;
+    asm("movl %%cr3, %%eax"
+        : "=a"(cr3));
+    return cr3;
+}
+
+void write_cr3(u32 cr3)
+{
+    asm volatile("movl %%eax, %%cr3" ::"a"(cr3)
+                 : "memory");
+}

Kernel/Arch/i386/CPU.h

@@ -278,15 +278,6 @@ void handle_crash(RegisterDump&, const char* description, int signal);
                                : "memory")
 #define memory_barrier() asm volatile("" :: \
                                           : "memory")
-
-inline u32 cpu_cr3()
-{
-    u32 cr3;
-    asm volatile("movl %%cr3, %%eax"
-                 : "=a"(cr3));
-    return cr3;
-}
-
 inline u32 cpu_flags()
 {
     u32 flags;
@@ -473,6 +464,9 @@ inline uintptr_t offset_in_page(const void* address)
     return offset_in_page((uintptr_t)address);
 }
 
+u32 read_cr3();
+void write_cr3(u32);
+
 class CPUID {
 public:
     CPUID(u32 function) { asm volatile("cpuid"

Kernel/VM/MemoryManager.cpp

@@ -49,11 +49,8 @@ MemoryManager& MM
 MemoryManager::MemoryManager()
 {
     m_kernel_page_directory = PageDirectory::create_kernel_page_directory();
-
     parse_memory_map();
-
+    write_cr3(kernel_page_directory().cr3());
-    asm volatile("movl %%eax, %%cr3" ::"a"(kernel_page_directory().cr3()));
-
     setup_low_identity_mapping();
     protect_kernel_image();
 }
@@ -266,7 +263,7 @@ Region* MemoryManager::region_from_vaddr(VirtualAddress vaddr)
 {
     if (auto* region = kernel_region_from_vaddr(vaddr))
         return region;
-    auto page_directory = PageDirectory::find_by_cr3(cpu_cr3());
+    auto page_directory = PageDirectory::find_by_cr3(read_cr3());
     if (!page_directory)
         return nullptr;
     ASSERT(page_directory->process());
@@ -475,16 +472,12 @@ void MemoryManager::enter_process_paging_scope(Process& process)
     InterruptDisabler disabler;
 
     current->tss().cr3 = process.page_directory().cr3();
-    asm volatile("movl %%eax, %%cr3" ::"a"(process.page_directory().cr3())
+    write_cr3(process.page_directory().cr3());
-                 : "memory");
 }
 
 void MemoryManager::flush_entire_tlb()
 {
-    asm volatile(
+    write_cr3(read_cr3());
-        "mov %%cr3, %%eax\n"
-        "mov %%eax, %%cr3\n" ::
-            : "%eax", "memory");
 }
 
 void MemoryManager::flush_tlb(VirtualAddress vaddr)
@@ -676,8 +669,7 @@ void MemoryManager::dump_kernel_regions()
 ProcessPagingScope::ProcessPagingScope(Process& process)
 {
     ASSERT(current);
-    asm("movl %%cr3, %%eax"
+    m_previous_cr3 = read_cr3();
-        : "=a"(m_previous_cr3));
     MM.enter_process_paging_scope(process);
 }
 
@@ -685,6 +677,5 @@ ProcessPagingScope::~ProcessPagingScope()
 {
     InterruptDisabler disabler;
     current->tss().cr3 = m_previous_cr3;
-    asm volatile("movl %%eax, %%cr3" ::"a"(m_previous_cr3)
+    write_cr3(m_previous_cr3);
-                 : "memory");
 }