Kernel: Enable PAE (Physical Address Extension)

Introduce one more (CPU) indirection layer in the paging code: the page
directory pointer table (PDPT). Each PageDirectory now has 4 separate
PageDirectoryEntry arrays, governing 1 GB of VM each.

A really neat side-effect of this is that we can now share the physical
page containing the >=3GB kernel-only address space metadata between
all processes, instead of lazily cloning it on page faults.

This will give us access to the NX (No eXecute) bit, allowing us to
prevent execution of memory that's not supposed to be executed.

1 files changed, 31 insertions, 10 deletions

diff --git a/Kernel/VM/PageDirectory.cpp b/Kernel/VM/PageDirectory.cpp
index e25c6e9119..729cf4c9be 100644
--- a/Kernel/VM/PageDirectory.cpp
+++ b/Kernel/VM/PageDirectory.cpp
@@ -24,7 +24,17 @@ RefPtr<PageDirectory> PageDirectory::find_by_cr3(u32 cr3)
 PageDirectory::PageDirectory(PhysicalAddress paddr)
     : m_range_allocator(VirtualAddress(0xc0000000), 0x3f000000)
 {
-    m_directory_page = PhysicalPage::create(paddr, true, false);
+    m_directory_table = PhysicalPage::create(paddr, true, false);
+    m_directory_pages[0] = PhysicalPage::create(paddr.offset(PAGE_SIZE * 1), true, false);
+    m_directory_pages[1] = PhysicalPage::create(paddr.offset(PAGE_SIZE * 2), true, false);
+    m_directory_pages[2] = PhysicalPage::create(paddr.offset(PAGE_SIZE * 3), true, false);
+    m_directory_pages[3] = PhysicalPage::create(paddr.offset(PAGE_SIZE * 4), true, false);
+
+    table().raw[0] = (u64)m_directory_pages[0]->paddr().as_ptr() | 1;
+    table().raw[1] = (u64)m_directory_pages[1]->paddr().as_ptr() | 1;
+    table().raw[2] = (u64)m_directory_pages[2]->paddr().as_ptr() | 1;
+    table().raw[3] = (u64)m_directory_pages[3]->paddr().as_ptr() | 1;
+
     InterruptDisabler disabler;
     cr3_map().set(cr3(), this);
 }
@@ -33,7 +43,26 @@ PageDirectory::PageDirectory(Process& process, const RangeAllocator* parent_rang
     : m_process(&process)
     , m_range_allocator(parent_range_allocator ? RangeAllocator(*parent_range_allocator) : RangeAllocator(VirtualAddress(userspace_range_base), kernelspace_range_base - userspace_range_base))
 {
-    MM.populate_page_directory(*this);
+    // Set up a userspace page directory
+
+    m_directory_table = MM.allocate_supervisor_physical_page();
+    m_directory_pages[0] = MM.allocate_supervisor_physical_page();
+    m_directory_pages[1] = MM.allocate_supervisor_physical_page();
+    m_directory_pages[2] = MM.allocate_supervisor_physical_page();
+    // Share the top 1 GB of kernel-only mappings (>=3GB or >=0xc0000000)
+    m_directory_pages[3] = MM.kernel_page_directory().m_directory_pages[3];
+
+    table().raw[0] = (u64)m_directory_pages[0]->paddr().as_ptr() | 1;
+    table().raw[1] = (u64)m_directory_pages[1]->paddr().as_ptr() | 1;
+    table().raw[2] = (u64)m_directory_pages[2]->paddr().as_ptr() | 1;
+    table().raw[3] = (u64)m_directory_pages[3]->paddr().as_ptr() | 1;
+
+    // Clone bottom 8 MB of mappings from kernel_page_directory
+    table().directory(0)[0].copy_from({}, MM.kernel_page_directory().table().directory(0)[0]);
+    table().directory(0)[1].copy_from({}, MM.kernel_page_directory().table().directory(0)[1]);
+    table().directory(0)[2].copy_from({}, MM.kernel_page_directory().table().directory(0)[2]);
+    table().directory(0)[3].copy_from({}, MM.kernel_page_directory().table().directory(0)[3]);
+
     InterruptDisabler disabler;
     cr3_map().set(cr3(), this);
 }
@@ -57,11 +86,3 @@ void PageDirectory::flush(VirtualAddress vaddr)
     if (this == &MM.kernel_page_directory() || &current->process().page_directory() == this)
         MM.flush_tlb(vaddr);
 }
-
-void PageDirectory::update_kernel_mappings()
-{
-    // This ensures that the kernel virtual address space is up-to-date in this page directory.
-    // This may be necessary to avoid triple faulting when entering a process's paging scope
-    // whose mappings are out-of-date.
-    memcpy(entries() + 768, MM.kernel_page_directory().entries() + 768, sizeof(PageDirectoryEntry) * 256);
-}