kernel/cpu/gdt: add safety comments

Missing safety comments were missing in GDT code. Adding a couple of
checks to validate the safety requirements.

Signed-off-by: Thomas Leroy <[email protected]>

kernel/src/cpu/gdt.rs

@@ -80,7 +80,18 @@ impl GDT {
 
         let tss_entries = &self.entries[idx..idx + 1].as_mut_ptr();
 
+        // SAFETY:
+        // For add():
+        //   - idx and idx + size_of::<GDTEntry>() don't overflow isize.
+        //   - the borrow checker guarantees that self is still allocated
+        //   - self.entries[6:8] fits in self's allocation.
+        // For write_volatile():
+        //   - the borrow checker guarantees that self.entries is allocated
+        //   - alignment is checked inside
         unsafe {
+            assert_eq!(align_of_val(&tss_entries.add(0)), align_of::<GDTEntry>());
+            assert_eq!(align_of_val(&tss_entries.add(1)), align_of::<GDTEntry>());
+
             tss_entries.add(0).write_volatile(desc0);
             tss_entries.add(1).write_volatile(desc1);
         }
@@ -93,11 +104,12 @@ impl GDT {
     pub fn load_tss(&mut self, tss: &X86Tss) {
         let (desc0, desc1) = tss.to_gdt_entry();
 
-        unsafe {
-            self.set_tss_entry(desc0, desc1);
-            asm!("ltr %ax", in("ax") SVSM_TSS, options(att_syntax));
-            self.clear_tss_entry()
-        }
+        self.set_tss_entry(desc0, desc1);
+        // SAFETY: loading task register must me done in assembly.
+        // It's safe to do so as long as a global GDT is in use and still
+        // allocated, which is always our case.
+        unsafe { asm!("ltr %ax", in("ax") SVSM_TSS, options(att_syntax)) };
+        self.clear_tss_entry()
     }
 
     pub fn kernel_cs(&self) -> GDTEntry {