Kernel: Make ProcessPagingScope restore CR3 properly

Instead of restoring CR3 to the current process's paging scope when a
ProcessPagingScope goes out of scope, we now restore exactly whatever
the CR3 value was when we created the ProcessPagingScope.

This fixes breakage in situations where a process ends up with nested
ProcessPagingScopes. This was making profiling very fragile, and with
this change it's now possible to profile g++! :^)

Kernel/Process.cpp

@@ -734,7 +734,8 @@ int Process::do_exec(NonnullRefPtr<FileDescription> main_program_description, Ve
 #ifdef MM_DEBUG
  dbgprintf("Process %u exec: PD=%x created\n", pid(), m_page_directory.ptr());
 #endif
- ProcessPagingScope paging_scope(*this);
+
+ MM.enter_process_paging_scope(*this);
 
  Region* region { nullptr };
 
@@ -775,11 +776,10 @@ int Process::do_exec(NonnullRefPtr<FileDescription> main_program_description, Ve
  m_regions.append(move(executable_region));
 
  ArmedScopeGuard rollback_regions_guard([&]() {
- m_page_directory = move(old_page_directory);
  ASSERT(&current->process() == this);
- MM.enter_process_paging_scope(*this);
- executable_region = m_regions.take_first();
+ m_page_directory = move(old_page_directory);
  m_regions = move(old_regions);
+ MM.enter_process_paging_scope(*this);
  });
 
  loader = make<ELFLoader>(region->vaddr().as_ptr(), loader_metadata.size);

Kernel/VM/MemoryManager.cpp

@@ -682,10 +682,15 @@ void MemoryManager::dump_kernel_regions()
 ProcessPagingScope::ProcessPagingScope(Process& process)
 {
  ASSERT(current);
+ asm("movl %%cr3, %%eax"
+ : "=a"(m_previous_cr3));
  MM.enter_process_paging_scope(process);
 }
 
 ProcessPagingScope::~ProcessPagingScope()
 {
- MM.enter_process_paging_scope(current->process());
+ InterruptDisabler disabler;
+ current->tss().cr3 = m_previous_cr3;
+ asm volatile("movl %%eax, %%cr3" ::"a"(m_previous_cr3)
+ : "memory");
 }

Kernel/VM/MemoryManager.h

@@ -190,9 +190,13 @@ class MemoryManager {
  bool m_quickmap_in_use { false };
 };
 
-struct ProcessPagingScope {
- ProcessPagingScope(Process&);
+class ProcessPagingScope {
+public:
+ explicit ProcessPagingScope(Process&);
  ~ProcessPagingScope();
+
+private:
+ u32 m_previous_cr3 { 0 };
 };
 
 template<typename Callback>