make the GDT base address configurable

api/build.rs

@@ -15,13 +15,14 @@ fn main() {
         (31, 9),
         (40, 9),
         (49, 9),
-        (58, 10),
-        (68, 10),
-        (78, 1),
-        (79, 9),
+        (58, 9),
+        (67, 10),
+        (77, 10),
+        (87, 1),
         (88, 9),
         (97, 9),
         (106, 9),
+        (115, 9),
     ];
 
     let mut code = String::new();

api/src/config.rs

@@ -35,7 +35,7 @@ impl BootloaderConfig {
         0x3D,
     ];
     #[doc(hidden)]
-    pub const SERIALIZED_LEN: usize = 115;
+    pub const SERIALIZED_LEN: usize = 124;
 
     /// Creates a new default configuration with the following values:
     ///
@@ -72,6 +72,7 @@ impl BootloaderConfig {
             kernel_stack,
             boot_info,
             framebuffer,
+            gdt,
             physical_memory,
             page_table_recursive,
             aslr,
@@ -94,45 +95,46 @@ impl BootloaderConfig {
         let buf = concat_31_9(buf, kernel_stack.serialize());
         let buf = concat_40_9(buf, boot_info.serialize());
         let buf = concat_49_9(buf, framebuffer.serialize());
+        let buf = concat_58_9(buf, gdt.serialize());
 
-        let buf = concat_58_10(
+        let buf = concat_67_10(
             buf,
             match physical_memory {
                 Option::None => [0; 10],
                 Option::Some(m) => concat_1_9([1], m.serialize()),
             },
         );
-        let buf = concat_68_10(
+        let buf = concat_77_10(
             buf,
             match page_table_recursive {
                 Option::None => [0; 10],
                 Option::Some(m) => concat_1_9([1], m.serialize()),
             },
         );
-        let buf = concat_78_1(buf, [(*aslr) as u8]);
-        let buf = concat_79_9(
+        let buf = concat_87_1(buf, [(*aslr) as u8]);
+        let buf = concat_88_9(
             buf,
             match dynamic_range_start {
                 Option::None => [0; 9],
                 Option::Some(addr) => concat_1_8([1], addr.to_le_bytes()),
             },
         );
-        let buf = concat_88_9(
+        let buf = concat_97_9(
             buf,
             match dynamic_range_end {
                 Option::None => [0; 9],
                 Option::Some(addr) => concat_1_8([1], addr.to_le_bytes()),
             },
         );
 
-        let buf = concat_97_9(
+        let buf = concat_106_9(
             buf,
             match minimum_framebuffer_height {
                 Option::None => [0; 9],
                 Option::Some(addr) => concat_1_8([1], addr.to_le_bytes()),
             },
         );
-        let buf = concat_106_9(
+        let buf = concat_115_9(
             buf,
             match minimum_framebuffer_width {
                 Option::None => [0; 9],
@@ -189,6 +191,7 @@ impl BootloaderConfig {
             let (&kernel_stack, s) = split_array_ref(s);
             let (&boot_info, s) = split_array_ref(s);
             let (&framebuffer, s) = split_array_ref(s);
+            let (&gdt, s) = split_array_ref(s);
             let (&physical_memory_some, s) = split_array_ref(s);
             let (&physical_memory, s) = split_array_ref(s);
             let (&page_table_recursive_some, s) = split_array_ref(s);
@@ -203,6 +206,7 @@ impl BootloaderConfig {
                 kernel_stack: Mapping::deserialize(&kernel_stack)?,
                 boot_info: Mapping::deserialize(&boot_info)?,
                 framebuffer: Mapping::deserialize(&framebuffer)?,
+                gdt: Mapping::deserialize(&gdt)?,
                 physical_memory: match physical_memory_some {
                     [0] if physical_memory == [0; 9] => Option::None,
                     [1] => Option::Some(Mapping::deserialize(&physical_memory)?),
@@ -351,6 +355,8 @@ pub struct Mappings {
     pub boot_info: Mapping,
     /// Specifies the mapping of the frame buffer memory region.
     pub framebuffer: Mapping,
+    /// Specifies the mapping of the GDT.
+    pub gdt: Mapping,
     /// The bootloader supports to map the whole physical memory into the virtual address
     /// space at some offset. This is useful for accessing and modifying the page tables set
     /// up by the bootloader.
@@ -388,6 +394,7 @@ impl Mappings {
             kernel_stack: Mapping::new_default(),
             boot_info: Mapping::new_default(),
             framebuffer: Mapping::new_default(),
+            gdt: Mapping::new_default(),
             physical_memory: Option::None,
             page_table_recursive: Option::None,
             aslr: false,
@@ -404,6 +411,7 @@ impl Mappings {
             kernel_stack: Mapping::random(),
             boot_info: Mapping::random(),
             framebuffer: Mapping::random(),
+            gdt: Mapping::random(),
             physical_memory: if phys {
                 Option::Some(Mapping::random())
             } else {

common/src/lib.rs

@@ -8,13 +8,22 @@ use bootloader_api::{
     info::{FrameBuffer, FrameBufferInfo, MemoryRegion, TlsTemplate},
     BootInfo, BootloaderConfig,
 };
-use core::{alloc::Layout, arch::asm, mem::MaybeUninit, slice};
+use core::{
+    alloc::Layout,
+    arch::asm,
+    mem::{size_of, MaybeUninit},
+    slice,
+};
 use level_4_entries::UsedLevel4Entries;
 use usize_conversions::{FromUsize, IntoUsize};
 use x86_64::{
-    structures::paging::{
-        page_table::PageTableLevel, FrameAllocator, Mapper, OffsetPageTable, Page, PageSize,
-        PageTable, PageTableFlags, PageTableIndex, PhysFrame, Size2MiB, Size4KiB,
+    instructions::tables::{lgdt, sgdt},
+    structures::{
+        gdt::GlobalDescriptorTable,
+        paging::{
+            page_table::PageTableLevel, FrameAllocator, Mapper, OffsetPageTable, Page, PageSize,
+            PageTable, PageTableFlags, PageTableIndex, PhysFrame, Size2MiB, Size4KiB,
+        },
     },
     PhysAddr, VirtAddr,
 };
@@ -201,8 +210,20 @@ where
         .allocate_frame()
         .expect("failed to allocate GDT frame");
     gdt::create_and_load(gdt_frame);
+    let gdt = mapping_addr(
+        config.mappings.gdt,
+        u64::from_usize(size_of::<GlobalDescriptorTable>()),
+        4096,
+        &mut used_entries,
+    );
+    let gdt_page = Page::from_start_address(gdt).unwrap();
     match unsafe {
-        kernel_page_table.identity_map(gdt_frame, PageTableFlags::PRESENT, frame_allocator)
+        kernel_page_table.map_to(
+            gdt_page,
+            gdt_frame,
+            PageTableFlags::PRESENT,
+            frame_allocator,
+        )
     } {
         Ok(tlb) => tlb.flush(),
         Err(err) => panic!("failed to identity map frame {:?}: {:?}", gdt_frame, err),
@@ -459,6 +480,7 @@ where
 
         kernel_slice_start,
         kernel_slice_len,
+        gdt,
         context_switch_trampoline: trampoline_page.start_address(),
         context_switch_page_table,
         context_switch_page_table_frame,
@@ -488,6 +510,8 @@ pub struct Mappings {
     pub kernel_slice_start: u64,
     /// Size of the kernel slice allocation in memory.
     pub kernel_slice_len: u64,
+    /// The address of the GDT in the kernel's address space.
+    pub gdt: VirtAddr,
     /// The address of the context switch trampoline in the bootloader's address space.
     pub context_switch_trampoline: VirtAddr,
     /// The page table used for context switch from the bootloader to the kernel.
@@ -611,6 +635,7 @@ pub fn switch_to_kernel(
         ..
     } = page_tables;
     let addresses = Addresses {
+        gdt: mappings.gdt,
         context_switch_trampoline: mappings.context_switch_trampoline,
         context_switch_page_table: mappings.context_switch_page_table_frame,
         context_switch_addr: mappings.context_switch_addr,
@@ -645,6 +670,18 @@ pub struct PageTables {
 
 /// Performs the actual context switch.
 unsafe fn context_switch(addresses: Addresses) -> ! {
+    // Update the GDT base address.
+    let mut gdt_pointer = sgdt();
+    gdt_pointer.base = addresses.gdt;
+    unsafe {
+        // SAFETY: Note that the base address points to memory that is only
+        //         mapped in the kernel's page table. We can do this because
+        //         just loading the GDT doesn't cause any immediate loads and
+        //         by the time the base address is dereferenced the context
+        //         switch will be done.
+        lgdt(&gdt_pointer);
+    }
+
     unsafe {
         asm!(
             "mov rsp, {}; sub rsp, 8; jmp {}",
@@ -661,6 +698,7 @@ unsafe fn context_switch(addresses: Addresses) -> ! {
 
 /// Memory addresses required for the context switch.
 struct Addresses {
+    gdt: VirtAddr,
     context_switch_trampoline: VirtAddr,
     context_switch_page_table: PhysFrame,
     context_switch_addr: VirtAddr,