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 @@ private:
     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>