Identity-map GDT into kernel address space

Author: phil-opp

src/binary/gdt.rs

@@ -0,0 +1,32 @@
+use x86_64::{
+    instructions::segmentation,
+    structures::{
+        gdt::{Descriptor, GlobalDescriptorTable},
+        paging::PhysFrame,
+    },
+    VirtAddr,
+};
+
+pub fn create_and_load(frame: PhysFrame) {
+    let phys_addr = frame.start_address();
+    log::info!("Creating GDT at {:?}", phys_addr);
+    let virt_addr = VirtAddr::new(phys_addr.as_u64()); // utilize identity mapping
+
+    let ptr: *mut GlobalDescriptorTable = virt_addr.as_mut_ptr();
+
+    let mut gdt = GlobalDescriptorTable::new();
+    let code_selector = gdt.add_entry(Descriptor::kernel_code_segment());
+    let data_selector = gdt.add_entry(Descriptor::kernel_data_segment());
+    let gdt = unsafe {
+        ptr.write(gdt);
+        &*ptr
+    };
+
+    gdt.load();
+    unsafe {
+        segmentation::set_cs(code_selector);
+        segmentation::load_ds(data_selector);
+        segmentation::load_es(data_selector);
+        segmentation::load_ss(data_selector);
+    }
+}

src/binary/mod.rs

@@ -24,6 +24,7 @@ pub mod bios;
 #[cfg(feature = "uefi_bin")]
 mod uefi;
 
+mod gdt;
 /// Provides a frame allocator based on a BIOS or UEFI memory map.
 pub mod legacy_memory_region;
 /// Provides a type to keep track of used entries in a level 4 page table.
@@ -170,6 +171,18 @@ where
         }
     }
 
+    // create, load, and identity-map GDT (required for working `iretq`)
+    let gdt_frame = frame_allocator
+        .allocate_frame()
+        .expect("failed to allocate GDT frame");
+    gdt::create_and_load(gdt_frame);
+    match unsafe {
+        kernel_page_table.identity_map(gdt_frame, PageTableFlags::PRESENT, frame_allocator)
+    } {
+        Ok(tlb) => tlb.flush(),
+        Err(err) => panic!("failed to identity map frame {:?}: {:?}", gdt_frame, err),
+    }
+
     // map framebuffer
     let framebuffer_virt_addr = if CONFIG.map_framebuffer {
         log::info!("Map framebuffer");