Kernel: Add mechanism to identity map the lowest 2MB

Author: https://github.com/tomuta
Commit: https://github.com/SerenityOS/serenity/commit/841364b6096
Pull-request: https://github.com/SerenityOS/serenity/pull/2475
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bugaevc

Kernel/VM/MemoryManager.cpp

@@ -211,8 +211,6 @@ void MemoryManager::initialize()
 
 Region* MemoryManager::kernel_region_from_vaddr(VirtualAddress vaddr)
 {
-    if (vaddr.get() < 0xc0000000)
-        return nullptr;
     for (auto& region : MM.m_kernel_regions) {
         if (region.contains(vaddr))
             return &region;
@@ -318,6 +316,18 @@ OwnPtr<Region> MemoryManager::allocate_kernel_region(PhysicalAddress paddr, size
     return allocate_kernel_region_with_vmobject(range, *vmobject, name, access, user_accessible, cacheable);
 }
 
+OwnPtr<Region> MemoryManager::allocate_kernel_region_identity(PhysicalAddress paddr, size_t size, const StringView& name, u8 access, bool user_accessible, bool cacheable)
+{
+    ASSERT(!(size % PAGE_SIZE));
+    auto range = kernel_page_directory().identity_range_allocator().allocate_specific(VirtualAddress(paddr.get()), size);
+    if (!range.is_valid())
+        return nullptr;
+    auto vmobject = AnonymousVMObject::create_for_physical_range(paddr, size);
+    if (!vmobject)
+        return nullptr;
+    return allocate_kernel_region_with_vmobject(range, *vmobject, name, access, user_accessible, cacheable);
+}
+
 OwnPtr<Region> MemoryManager::allocate_user_accessible_kernel_region(size_t size, const StringView& name, u8 access, bool cacheable)
 {
     return allocate_kernel_region(size, name, access, true, true, cacheable);
@@ -665,7 +675,7 @@ void MemoryManager::unregister_vmobject(VMObject& vmobject)
 void MemoryManager::register_region(Region& region)
 {
     InterruptDisabler disabler;
-    if (region.vaddr().get() >= 0xc0000000)
+    if (region.is_kernel())
         m_kernel_regions.append(&region);
     else
         m_user_regions.append(&region);
@@ -674,7 +684,7 @@ void MemoryManager::register_region(Region& region)
 void MemoryManager::unregister_region(Region& region)
 {
     InterruptDisabler disabler;
-    if (region.vaddr().get() >= 0xc0000000)
+    if (region.is_kernel())
         m_kernel_regions.remove(&region);
     else
         m_user_regions.remove(&region);

Kernel/VM/MemoryManager.h

@@ -107,6 +107,7 @@ public:
     OwnPtr<Region> allocate_contiguous_kernel_region(size_t, const StringView& name, u8 access, bool user_accessible = false, bool cacheable = true);
     OwnPtr<Region> allocate_kernel_region(size_t, const StringView& name, u8 access, bool user_accessible = false, bool should_commit = true, bool cacheable = true);
     OwnPtr<Region> allocate_kernel_region(PhysicalAddress, size_t, const StringView& name, u8 access, bool user_accessible = false, bool cacheable = true);
+    OwnPtr<Region> allocate_kernel_region_identity(PhysicalAddress, size_t, const StringView& name, u8 access, bool user_accessible = false, bool cacheable = true);
     OwnPtr<Region> allocate_kernel_region_with_vmobject(VMObject&, size_t, const StringView& name, u8 access, bool user_accessible = false, bool cacheable = true);
     OwnPtr<Region> allocate_kernel_region_with_vmobject(const Range&, VMObject&, const StringView& name, u8 access, bool user_accessible = false, bool cacheable = true);
     OwnPtr<Region> allocate_user_accessible_kernel_region(size_t, const StringView& name, u8 access, bool cacheable = true);
@@ -143,6 +144,8 @@ public:
 
     PhysicalPage& shared_zero_page() { return *m_shared_zero_page; }
 
+    PageDirectory& kernel_page_directory() { return *m_kernel_page_directory; }
+
 private:
     MemoryManager();
     ~MemoryManager();
@@ -177,8 +180,6 @@ private:
     PageDirectoryEntry* quickmap_pd(PageDirectory&, size_t pdpt_index);
     PageTableEntry* quickmap_pt(PhysicalAddress);
 
-    PageDirectory& kernel_page_directory() { return *m_kernel_page_directory; }
-
     const PageTableEntry* pte(const PageDirectory&, VirtualAddress);
     PageTableEntry& ensure_pte(PageDirectory&, VirtualAddress);
 

Kernel/VM/PageDirectory.cpp

@@ -59,6 +59,7 @@ extern "C" PageDirectoryEntry boot_pd3[1024];
 PageDirectory::PageDirectory()
 {
     m_range_allocator.initialize_with_range(VirtualAddress(0xc0800000), 0x3f000000);
+    m_identity_range_allocator.initialize_with_range(VirtualAddress(FlatPtr(0x00000000)), 0x00200000);
 
     // Adopt the page tables already set up by boot.S
     PhysicalAddress boot_pdpt_paddr(virtual_to_low_physical((FlatPtr)boot_pdpt));

Kernel/VM/PageDirectory.h

@@ -52,6 +52,7 @@ public:
     u32 cr3() const { return m_directory_table->paddr().get(); }
 
     RangeAllocator& range_allocator() { return m_range_allocator; }
+    RangeAllocator& identity_range_allocator() { return m_identity_range_allocator; }
 
     Process* process() { return m_process; }
     const Process* process() const { return m_process; }
@@ -62,6 +63,7 @@ private:
 
     Process* m_process { nullptr };
     RangeAllocator m_range_allocator;
+    RangeAllocator m_identity_range_allocator;
     RefPtr<PhysicalPage> m_directory_table;
     RefPtr<PhysicalPage> m_directory_pages[4];
     HashMap<unsigned, RefPtr<PhysicalPage>> m_physical_pages;

Kernel/VM/RangeAllocator.h

@@ -90,6 +90,11 @@ public:
 
     void dump() const;
 
+    bool contains(const Range& range) const
+    {
+        return m_total_range.contains(range);
+    }
+
 private:
     void carve_at_index(int, const Range&);
 

Kernel/VM/Region.cpp

@@ -40,13 +40,14 @@
 
 namespace Kernel {
 
-Region::Region(const Range& range, NonnullRefPtr<VMObject> vmobject, size_t offset_in_vmobject, const String& name, u8 access, bool cacheable)
+Region::Region(const Range& range, NonnullRefPtr<VMObject> vmobject, size_t offset_in_vmobject, const String& name, u8 access, bool cacheable, bool kernel)
     : m_range(range)
     , m_offset_in_vmobject(offset_in_vmobject)
     , m_vmobject(move(vmobject))
     , m_name(name)
     , m_access(access)
     , m_cacheable(cacheable)
+    , m_kernel(kernel)
 {
     MM.register_region(*this);
 }
@@ -186,14 +187,14 @@ size_t Region::amount_shared() const
 
 NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, NonnullRefPtr<VMObject> vmobject, size_t offset_in_vmobject, const StringView& name, u8 access, bool cacheable)
 {
-    auto region = make<Region>(range, move(vmobject), offset_in_vmobject, name, access, cacheable);
+    auto region = make<Region>(range, move(vmobject), offset_in_vmobject, name, access, cacheable, false);
     region->m_user_accessible = true;
     return region;
 }
 
 NonnullOwnPtr<Region> Region::create_kernel_only(const Range& range, NonnullRefPtr<VMObject> vmobject, size_t offset_in_vmobject, const StringView& name, u8 access, bool cacheable)
 {
-    auto region = make<Region>(range, move(vmobject), offset_in_vmobject, name, access, cacheable);
+    auto region = make<Region>(range, move(vmobject), offset_in_vmobject, name, access, cacheable, true);
     region->m_user_accessible = false;
     return region;
 }
@@ -268,8 +269,12 @@ void Region::unmap(ShouldDeallocateVirtualMemoryRange deallocate_range)
         dbg() << "MM: >> Unmapped " << vaddr << " => P" << String::format("%p", page ? page->paddr().get() : 0) << " <<";
 #endif
     }
-    if (deallocate_range == ShouldDeallocateVirtualMemoryRange::Yes)
+    if (deallocate_range == ShouldDeallocateVirtualMemoryRange::Yes) {
+        if (m_page_directory->range_allocator().contains(range()))
             m_page_directory->range_allocator().deallocate(range());
+        else
+            m_page_directory->identity_range_allocator().deallocate(range());
+    }
     m_page_directory = nullptr;
 }
 

Kernel/VM/Region.h

@@ -95,6 +95,9 @@ public:
     bool is_user_accessible() const { return m_user_accessible; }
     void set_user_accessible(bool b) { m_user_accessible = b; }
 
+    bool is_kernel() const { return m_kernel || vaddr().get() >= 0xc0000000; }
+    void set_kernel(bool kernel) { m_kernel = kernel; }
+
     PageFaultResponse handle_fault(const PageFault&);
 
     NonnullOwnPtr<Region> clone();
@@ -178,7 +181,7 @@ public:
     Region* m_prev { nullptr };
 
     // NOTE: These are public so we can make<> them.
-    Region(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmobject, const String&, u8 access, bool cacheable);
+    Region(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmobject, const String&, u8 access, bool cacheable, bool kernel);
 
     void set_inherit_mode(InheritMode inherit_mode) { m_inherit_mode = inherit_mode; }
 
@@ -211,6 +214,7 @@ private:
     bool m_cacheable : 1 { false };
     bool m_stack : 1 { false };
     bool m_mmap : 1 { false };
+    bool m_kernel : 1 { false };
     mutable OwnPtr<Bitmap> m_cow_map;
 };